User blog comment:Tekkphreke/Shikai and Bankai/@comment-1771627-20131224154347

I think the mistake is considering Shikai and Bankai different thing, independent in which form they are, they are training with a Zanpakutou anyways. Strengthening one is strengthening the other, what happens is that both forms differ in appearance. For example, of course Renji uses different skills while using Shikai and Bankai, given how the sword changes and requires other types of mastery.

About the final question, I will agree with Xil. All shinigami have inside them the potential to be as powerful as Yamamoto, but what lacks in all is his experience/iron will/etc. Compare Ryujin Jakka and Tobiume. Both can shoot fire, obviously on different levels, but if Momo were to be on the same level of Yamamoto, I don't doubt Tobiume would be capable of spamming hells or the like.