I don't think the drop-off from Winston/Mariota to him is as big as the media would have us believe. I know he was mediocre when he came to Alumni this year, but hear me out.
Grayson is not as tall as Winston & Mariota, but he is still 6-2 220 and his hands are bigger than Winston's and Mariota's.
One of the major criticisms of Grayson is that he has tunnel vision, but Winston and Mariota are no better in this regard. Each of their #1 targets had at least double the number of receptions than the #2 receiver and all three had completion percentages in the mid-60s.
Critics say Grayson's YPA is inflated by his receivers' high YAC numbers. That is unfair considering the WR talent at FSU & Oregon versus Colorado State.
Scouts also fault Grayson's mechanics, but he proved that he can learn new mechanics during the 2014 off-season.
I don't think Grayson is a top 15 pick, but if you need a QB and he's available in the bottom half of the first round, you roll the dice on him.