I love Grayson as a collegiate player. But, frankly, it would be surprising if any team falls in love with him enough to guarantee him a spot in the first round. Chad Ford has him loosely rated in the 20-30 range, Draft Express lists him at 31 (i.e., first pick of 2nd round), draft update has him at 25, and CBS's first round mock does not include Grayson. While I personally think Allen will end up becoming a solid NBA player, this is not like a lot of our past early entrants who were virtually guaranteed of going first round. From what I've read to date, the odds are he'd have a decent chance of making the first round; but that he's the type of controversial prospect who could certainly tumble out of the first round if a few teams decide to go in a different direction.
Now, with that said, can Grayson markedly improving his drafty stock by staying? Probably no in view of who we're bringing in next year. But, I do think he certainly could improve his chances of actually making a roster even if he does fall into the 2nd round -- as opposed to spending most of his rookie year in the D-league, which is what I think will happen either way.
I'm fine with either decision -- and, if you're of the view that there's no point of ever staying in college when someone's willing to draft you, then I think it probably makes sense to declare. But this is not a Jabari Parker-like case (or even Austin Rivers, Gerald Henderson, or Kyle Singler post NCAA MOP) where he's definitely turning down guaranteed millions by staying. In the end of the day, I think he's probably in the Kyle Singler boat, where he's a good enough overall player that he'd make a roster even if he has a bit of a subpar jr. year and falls into the 2nd round.