The point of the Rule of Two was to attack the Jedi using the tools of deception and secrecy, rather than war and combat, and to encourage the apprentice to overthrow his master using those same methods, thereby ensuring that the order became more effective over time to perform in that specific capacity. The idea that it was intended for successive generations to actually become stronger in the Force/more adept in combat is a huge misconception on the parts of some, and blatant misinformation being handed out by others. Strength in the Force is hugely important in determing how powerful and adept in combat a Force User is and Bane is one of the naturally most powerful people we've ever come across (the fulfillment of the most important Sith prophecy that specifically spoke of his incredible power), so the idea that the order would have produced more powerful Force Users over time is simply idiotic.
This approach of completely disregarding feats simply because they happened on nexuses is simply an example of an incredibly unrefined methodology. I don't think there's even a single example of a nexus that has characters performing at close to twice the level they usually perform at (roughly speaking) and many examples where they perform at best marginally better (e.g. Dooku on Vjun against Yoda where he is still beaten despite numerous situational advantages). And in Bane's case specifically and his destruction of the rakatan temple on Lehon, there's absolutely nothing in the narration to suggest that the amp was anywhere near as high as people would like to claim. He simply makes a casual observation that the Force was really strong on the world, nothing that would indicate that his powers had grown to incredible new heights.
It would be absurd to suggest that his powers were even doubled, and the temple was so large that you could probably reduce it tenfold and it would still be an incredible feat of telekinesis.
__________________ "Happiness is a lie. Life is horror. The light is always dying all across the universe. The last star will flicker out someday, when it does, all that remains is shadow. And I will be its king!"'-Amahl Farouk
It's said in TPM scrapbook that in the rule of two the successor of Dark Lord is more powerful than the previous successor.
__________________ "Vader's pulse and breathing were machine-regulated, so they could not quicken; but something in his chest became more electric around his meetings with the Emperor; he could not say how. A feeling of fullness, of power, of dark and demon mastery -- of secret lusts, unrestrained passion, wild submission -- all these things were in Vader's heart as he neared his Emperor. These things and more."
More powerful in the sense of being more skilled in the methodology that the Rule of Two built itself around. You need to stop thinking of the ROT Sith as fighters but as schemers. A more powerful schemer is someone who has greater skill at orchestrating schemes.
Um, no. In typical Sith fashion, the only intended way to advance is to actively kill your master, in a duel. These are still Sith and they still function like Sith always have. Always attaining greater power in the Force and continued strengthening of the Sith. The Rule of Two wasn't established to scheme, it was established to end the usual scheming that only weakened the Sith time and time again, as well as concentrate all power and Dark Side knowledge into two individuals at a time. Which is why Bane specifically speaks of Zannah's power eclipsing his as the only time she should kill him, and not say her wit and ability to defeat him in chess in six moves.