By the time the combine is done I think he will definitely be a top 20 pick. The guy is an amazing athletic talent with a ton of versatility and he has proven he can get it done on the field against top competition, players like that who play his position don't last very long on draft day.
There seems to be a crapload of quality DTs worth top 20 picks, yet not that many teams in the 10-24 range at all who need DTs that badly (or at the very least are likely to take a DT). This is probably gonna cause at least one or two of Poe, Brockers, Cox, and Still to drop into bargain range in the mid-20s barring trade-ups. Not to mention Jerel Worthy, Brandon Thompson, Alameda Ta'aumu, and Kendall Reyes who are worth first round picks but might fall into round two.
Cox though I think finds his way into the top 20 after his combine performance. He has quite good film too and it sounds like a great competitive attitude. He outperformed a lot of linebackers with his workout numbers all around - even in agility tests.