Part 1
That's now how it always works, like, at all.At times, Force sensitives are blatantly better at some Force abilities than others, no matter that power gap. It's how Mara Jade was touted as sporting superior Force senses to Luke, despite the latter being significantly more powerful; how Corran Horn's usage of tutaminis and illusion abilities surpasses nearly everyone in the NJO, despite a number of people being his superior; it's how some guys can have all the knowledge, understanding, mastery and strength in the Force, if not more, than the next inferior being, but still be unable to utilize certain abilities [E.I. Darth Bane and Darth Plagueis having no talent for Sith Sorcery]. If what you're saying is true, Bane, for example, shouldn't have had any trouble mastering sorcery, or Luke senses should be far above Mara's own - but as text indicate, that simply isn't the case.
So, while I generally agree Tenebrous should be capable of replicating the basic Force abilities of his predecessors plus some, mainly telekinetic prowess and the like, I wouldn't say the more esoteric powers that aren't common to Jedi and Sith [ Force Phantoms for example] shouldn't fall in this niche, thus, I don't think you can properly scale off of it.
Either with scaling, Tenebrous' implied showings are inferior.
Yes, outliers exist. But unless you have something better than “I don’t think that Tenebrous can blah blah blah…” to substantiate the restrictions you are putting on his ability to use esoteric Force powers, you can suck his Bith cock. You have no evidence that Tenebrous lacks the talent for any of these abilities, nor have you evidence that some of the intermediary Banites who possessed these abilities had an unnatural aptitude for them that most other Force users lacked. The reason they aren’t common amongst Sith and Jedi because the Banite Order is a little more advanced than your average Jedi Knight or Sith Lord, and as such, have more advanced standards for what is the norm in terms of Force knowledge and mastery.
Tenebrous’ only canonical limitation in terms of lacking talent for something is Sith Sorcery. And note how simply lacking talent doesn’t mean he is incapable of Sorcery entirely. It simply means he learns and masters it slowly, and that it takes him significantly more effort to attain the same level of proficiency with it as a talented person. This is the case with any skill, really, and even Star Wars we have examples of some people having a higher Force potential than others, and thus having more talent to use the Force, whereas some people lack that talent, yet are still able to use the Force well and in some cases attain an equal level of ability with the talented person (Anakin vs Obi-Wan on Mustafar comes to mind immediately, where Kenobi is able to stalemate Vader in the Force despite his far lesser talent, which was so little in fact that he was sent off to the Jedi Agricultural Corps and was deemed to never become a Jedi until he met Qui-Gon, yet decades later he is among the legends of the Order’s entire history).
Why am I bringing this up? Well, I do indeed have proof that Tenebrous should be capable of some high level Sorcery (and high level Force abilities in general) despite his lack of talent for it. First of all, both Plagueis and Bane, as you’ve noted, lacked the talent for Sith Sorcery, yet we see both of them pulling off Sorcery feats on the fly:
Like a feline, Sidious leapt from the scree, his curled fingers aimed for Plagueis. But instead of vising themselves around the Muun’s slender neck, his hands went through thin air and met each other, leaving him to collapse face-first atop the outcropping. Off to one side he heard his Master laugh in scorn. Either Plagueis had moved faster than Sidious could discern or, worse yet, he had never been there to begin with.
“So easily tricked,” Plagueis said, confirming the latter. “You waste my time. More of this and the dark side will never take an interest in you.”
--Darth Plagueis
With his mind he tried to project an image of his hook-handled weapon resting just beneath his empty fingers. His apprentice didn't move. She stayed in her defensive stance, her brow furrowing as she weighed her chances.
--Darth Bane: Dynasty of Evil
And yes, Force Illusions are identified under the umbrella of Sith Sorcery, in case you’re wondering:
But sorcery had been employed less to extend life than to create illusions, fashion beasts, and resurrect the dead.
--Darth Plagueis
Bane’s case is particularly interesting. He manages a Force Illusion, while drugged and as a consequence having his connection to the Force weakened, in the midst of combat, and successfully fools a more powerful Force user than him. His lack of talent didn’t seem to hinder him there – it’s only the really, really advanced stuff that he couldn’t properly understand, like Zannah’s Spells of Madness (which, for the record, are also more complicated than any Force ability even Vitiate ever showed in combat). Given the Banite mentality that you must master everything in order to fight and surpass your Master one day, as well as pass all your knowledge onto the next generation, I find it unlikely that Tenebrous wouldn’t have mastered Sith Sorcery as far as he could go with it – that is, being able to do Sorcery on the fly in the midst of combat and have it affect equally or more powerful Force users successfully, which is the level of proficiency Bane achieved, and Tenebrous has a far higher base potential and intrinsic aptitude than Bane.
Then there’s Plagueis, who also manged an Illusion casually. It should be noted that all that Plagueis knew came from Tenebrous, so we know for a fact that Tenebrous is also capable of Sorcery on the fly, casually, even if you don’t buy the Bane logic above. To that fact we can add another and Tenebrous becomes an even more impressive sorcerer; the fact that Tenebrous (in comparison to his overall Force knowledge and mastery) hadn’t actually taught Plagueis very much at all:
Instead of actually training his doltish apprentice, Tenebrous had flattered Plagueis' mysticism while pricking his insecurities, sending him off on one useless, doomed-to-fail mission after another.
[…]
Hmm-perhaps he should have invested some time in actually training the foolish Muun. Tapping Plagueis' Force powers would be more entertaining if they weren't so stunted from disuse.
--The Tenebrous Way
So Tenebrous, according to his own words, hadn’t “actually trained” Plagueis. While this is hyperbole, as Plagueis does muse about his training sessions (Sith Sorcery included) with Tenebrous, it does speak of the level of knowledge Tenebrous withheld from his apprentice, if what he taught to Plagueis doesn’t constitute as “actual training” in his eyes. So logically this would extend to Sith Sorcery as well, given that if what Plagueis showed regarding Illusions was all that Tenebrous knew himself, then that would mean that the latter would have given all of his illusory knowledge to the former, which would constitute as actual training, but that cannot be given what Tenebrous has stated. So it stands to reason that Tenebrous is a far mightier sorcerer than either Bane or (early-novel) Plagueis and his Illusions are top tier. He may even be proficient in other sorcerous Force powers, although that is mere speculation. Not that it matters, since the point here is to prove that even if he is lacking in talent in certain areas, or that if some of his predecessors were very attuned to specific Force powers (all of which remain completely baseless assumptions on your part), Tenebrous can still attain a very high level of mastery in said areas through sheer determination and intensive practice and study, possessing one of, if not the greatest mind out of all Sith Lords in terms of sheer brain power and IQ.
However, I am not done yet. Tenebrous not only has the capacity to make up for his lack of talent with sheer determination, but his own aptitude and talent for esoteric Force abilities is in itself higher than average, so your assumptions make even less sense. For starters:
More than a century before, when Tenebrous had been but a Sith apprentice himself, the magnificent computational power of his Bith brain had led him far beyond the simplistic Force studies imposed on him by his Master.
--The Tenebrous Way