Might be ignorance on my part, but I can't imagine pushing with the heels in any phases of the pedaling cycle. Although the advent of carbon soles has done well to help spread pedaling pressure across a wider area of the foot, the balls of the feet are still the main contact point. This you can feel no matter how stiff the soles.

FWIW, I've found the SIDI mega design to not only be slightly wider than the standard SIDI width, but it also gains some of that extra width from increased vertical volume as well. Additionally I do not believe or accept the idea that a critical component such as a cycling shoe will or should break-in to proper fit and function. With modern non-leather uppers I expect it to fit right from the first try, and to maintain that fit throughout its useful life.