I don't think the level of competition should really be an issue with WR. QB's sure but skill players I just can't get behind that. I believe Moss and Owens both had these same questions when they were drafted. Moss had some other baggage at the time which hurt his stock but still had the competition question attached to him.
If the player can run, jump and catch I don't believe the competition is really going to matter. For a QB it is a little bit different as you can get away with mistakes that you wouldn't normally get away with in other conferences but for a WR there is very little that you can scrutinize there.
You can say that the numbers would come down and they normally do even from players who are at a higher level of competition. I wouldn't be worried about competition when drafting a WR I am more looking at how well they cut, run routes and catch the ball. I would put the competition last or not even in consideration on determining which WR to draft over another. Competition matters when you are comparing stats but when comparing players on who you want to draft I don't think it matters at the skill positions.