I honestly don't know a lot about Tannehill. I don't watch a lot of College Football, but was immediately put off by hearing that he was a converted WR with only a year and a half experience.
This link is to a pretty interesting Matrix on Football Outsiders. Check out last years as well (linked in the article). I still don't know how I feel - Tannehill does grade out better than some of the 1st rounders drafted last year though, and I think this is interesting stuff.