If my ferrothorn is wearing a power band while I EV train in defense do I get the boost?

No. A power item gives 4 EV's to the stat it affects regardless of what it battles. This is added to what it get from the pokemon defeated. Pokerus doubles it to 8. The absolute maximum a pokemon can get is 14. But it needs to have the power item, pokerus and the pokemon defeated has to give 3 ev's normally. So a power item mixed with pokerus taking on a pokemon that give 1 ev you would get 10 per battle for that stat.