Ignoring salary rules etc I'd trade Tatum straight up for Davis. I sometimes waver a little on that, mainly due to contract and durability, but I realize a lot of that's just the homer in me.
I have a hard time imagining what I'd include on top of Tatum to land him. If you assume you lose Smart this offseason it's worth including him. Maybe one of our lesser 1sts. Definitely not Brown on top.
I don't know, maybe I'm getting ahead of myself but the kid looks incredible so far, on a (relatively) cheap deal, plus RFA and Bird rights. Hard to pass up.