Was it a typo that both of those are "road shoes"?
I hate it when I do that.
I should have compared apples to apples, say Sidi Genius 5 Pro Carbon Mega Road vs Sidi Dominator 5 Mega MTB, each going for same price, 614g for the road shoes, 696g for the mtb shoes. That's about three ounces worth of difference, which is significant, although somewhat negated by using eggbeaters.