Infernape does outspeed both Mega X and Mega Y so that's true, and if you predict the Mega Evo, you will OHKO Charizard Y, not sure about X though. The only downside is Stone Edge's accuracy.
EDIT: Did some calcs
252 Atk Choice Band Infernape Stone Edge vs. 4 HP / 0 Def Mega Charizard Y...