If I had a Cinderace with a Power Anklet and 255 EVs in Attack, and then I battled a Sharpedo and KOed it, would I still get the 8 Speed EVs from Power Anklet even though I didn't gain any in the attack stat?
If a Pokémon holding a Power item gains experience from a battle, the Power item awards it additional EVs in a specific stat (in addition to the EVs it would normally earn).
I believe it would gain the Speed EVs.
Source
Hope I helped!