The 4 EVs from the Power items are added before the Pokerus doubles. So you get 1 from the Bellsprout plus 4 from the Power item gives 5. Double that for 10. There are 5 Pokemon per horde, so you get 10, 5 times. Which is 50.
That's why it's so much easier. 5 battles and you're practically maxed out. Get a Pokemon that can learn Sweet Scent (if used in tall grass it initiates a horde battle) and a move that hits all 5 and you're good to go.
Currently playing MH3U, FE:A and Pokemon X Remember kids, there's no kill like overkill.