I believe the characterization and acting is important. Obviously it is. But appearance is as well. The point of Bane's size is to make him menacing. I saw a comment somewhere else that said: What if Drago was the same size (if not shorter) than Rocky. It would have a TOTALLY different feel to it.
And that's the same point here. Of course the acting is important and doing Bane justice as a character, but he SHOULD tower over Batman because it would make for better visuals.
The mask change is the least of the worries. The mask change is similar to the Joker wearing make-up (instead of being perma).
But take the Joker...make him wear yellow instead of purple, and make him a short fat guy...now? does it feel like the Joker?
"Oh but, he laughs, and is violent, and tells bad jokes, and murders people and is a criminal genius, so the characterization is perfect!"
...see what I'm trying to say here?
--dk7