The power items, for EV training. They just double the number of EVs gained, right? So, if I fight a Starly, I get 2 Speed EVs instead of just one, right?
No...
Depending on the Power Item, it adds 4 EV points to a certain stat at the end of every battle, regardless of the Pokemon fought. Only the Macho Brace actually doubles the amount of EVs gained from a Pokemon after a battle.