It boosts all of these moves:
Bullet Punch Comet Punch Dizzy Punch Drain Punch DynamicPunch Fire Punch Focus Punch Hammer Arm Ice Punch Mach Punch Mega Punch Meteor Mash Shadow Punch Sky Uppercut ThunderPunch
So yes, Sky uppercut gets a boost. http://bulbapedia.bulbagarden.net/wiki/Iron_Fist_(ability)