You could always argue that with the addition of some muscle these guys could increase power/speed. That is not always the case, and IMO, probably not with Dave Z. He has found his optimum power/weight ratio at his listed racing weight.
Some people gain muscle and get faster, some gain it and slow down. It's highly individual and wopuld be hard to answer definitively.
DZ may be able to put on a few pounds of muscle and go .001mph faster, but evidently is doesn't benefit him or he probably would have done it.