I have a shiny sylveon and it has a poor defence and an okay speed stat and it still has more room for evs. Will pokerus double the amount of evs my sylveon gets?
Sadly No it won't.
"In Generation VI, Pokérus does not affect effort values gained during Super Training."
Sources: http://bulbapedia.bulbagarden.net/wiki/Pokérus
http://www.marriland.com/forums/pokemon-6th-generation/pokemon-x-y/449424-pokerus-and-super-training
Edit: This question has also been asked before click here for details.
Hope I helped :)