Power items give +4 in the specified EV it is (Power lens gives +4 spec atk, power anklet gives +4 speed EV, power bracelet gives +4 atk, etc). They boost +4 EVs to the specified stat from ANY wild pokemon you fight.
So, if you were to fight a wild gastly, you would get 1 Spec atk EVs by default PLUS an additional 4 from a power lens resulting in +5 spec atk EVs. If you were to have Pokerus too, it would result in 10 EVs. Macho brace is alright, but if you have the patience and Battle Points, get the power set and EV training will be nothing.