I believe aizen worded it something like "What they dont know is that it can be awakened for a few short seconds with someone having about twice the reiastu level of a captain." or so..
1.75x captain level and 2.5x captain level could be an insanely huge difference.. plus its a rough estimate.. It could be really something only 1.5x and 4-5x depending on how they actually rated it. Im just saying its likely that its around 1.75x and, i guess ill say 3x now.. I never said this was how it was, just an extremely rough estimate considering the situations. When you consider how strong they are.. thats is a huge difference.
Renji is not captain-level. He may have obtained bankai, but if he were to actually become a captain he would be by far the weakest. By a good amount... Hes at like ikkaku's level.. And ikkaku got his towered destroyed.. (yeah, he didnt use his bankai, but still) If they were to become captains, they would definitely lower the power of the 13 captains, overall, by a good bit.
Kubo might have messed up with some of this... Seeing as he made aizen obviously a lot stronger than ichigo. Although, he really might not have. But i consider it like this. If someone is 1.5x stronger than you, smarter than you, more experienced, better, and even trained when you are not, its obvious that the other guy doesnt stand a chance. He has him beat in every level, the weaker one would have to have extreme luck to survive.. Its pretty much that. Stronger guy = aizen, weaker = ichigo.