I have watched Thigpen for many years. I saw him regularly at Indiana after he came out of high school in Detroit. (Normally Indiana lands Detroit prep stars only if Michigan and MSU pass on him). He was always excellent at returning kicks and if I recall correctly used his blazing speed at running back in the base offense and not split out. I don't recall him in the slot at all. Now, since then he played in Canada for multiple years and played in Miami for a year. Historically, I have never seen the route running ability, awareness, or instincts at all in the slot.
Now, with our good coaches, Miami put him in the slot and I think he made a couple of plays last year but I don't think it was anything at all due to his routes. I think he was schemed to make the plays. Hopefully, he can radically improve his awareness in the slot because he does have great long speed and really good quickness. He can be a weapon. If he never figures out how to win his one-on-one battles and get separation with his routes though, I think you need someone else in the slot you can find on 3rd down comfortably.