I don't think the 5 WRs are set by any means. The only proven guys on the roster at the moment are Cobb, Nelson and Adams. Abbrederis needs to prove he can stay healthy before the team should actually count on him and that's not to mention that he hasn't actually proven he can play at an NFL level (concussion history and then he gets injured pretty much right away in the NFL). Janis is all potential at this point and I don't really know enough about White other than me not knowing much about him means he probably has shown much to this point (not saying he can't, just that he hasn't).
So, yeah, I would imagine that WR is a position Thompson would look at. This year's draft is supposed to be pretty deep at WR so I wouldn't be shocked if Thompson goes WR in the second or third round.