Any other time in the Banite Order is what I meant.
If you really think about it Jerec is really impressive. He was just this random Jedi bookworm with no special potential or relationship to the Force like the Skywalkers, not noted to be a prodigy and not coming from any impressive "lineage" of training, like Yoda>Dooku>Qui-Gon>Obi-Wan>Anakin.
Now take someone like Dooku, who was noted by Yoda to be a prodigy and one of the most powerful and celebrated members of the Jedi. And after some 6-7 decades as a Jedi he has another decade of apprenticeship under Sidious himself, after which even Yoda remarked he grew more powerful.
Jerec on the other hand only had whatever scraps of knowledge he found on his expeditions and whatever scraps of knowledge he got as an Inquisitor under Palpatine, which we know was purposely very limited. Palpatine even cut off Jerec's access to the Dark Side Compendium. Why do that if he didn't consider him a legitimate theat?
In spite of that he still managed to become about as powerful as Dooku, or comparable(but weaker) to Vader. But what if he had spent one or two decades as a proper apprentice in the Banite line and had access to their main teachings?