No, I haven't seen Davis in person, and I'm not making any big proclamations about what he'll be in college. For the record, though, ESPN, which used to be your favorite ratings site, has him as the 21st best center, and 247 has him as a 4 star (albeit the second to last one and at about 130 overall) and the 17th best center. He was also 2nd in Michigan Mr. Basketball voting. Presumably these people have seen him in person and liked what they say well enough.
And I don't think isolation post-ups would be his strong suit, because there you need to generate your own shot. Maybe he can develop some moves, an over the shoulder hook, but A PnR or PnPop allow him to generate looks off of action. Big body, good hands, nice finishing touch, able to shoot, smart player, but lacks quickness or explosion -- I maintain that a PnR/PnP offense is better for a player of that skill set than just dumping it into the post and telling him to go to work.