Whitner, like Mayo who was a surprise high pick, came in and became immediate starters/impact players. Barwin is coming in with tons of hype and likely will not live up to it, especially in his first three years.
Mav, let's separate out the issue of where Barwin is likely to go from where he deserves to go.
Let's assume that Barwin's true value is a late 2nd/early 3rd round pick. I don't agree with that, but let's assume it. I would argue that he is likely to go much higher than that based on his athleticism, versatility, and rapid progression on defense. Even if we rate him accurately, teams will get carried away. That's why I say I think he is likely to be a late 1st/early 2nd round pick, regardless of where the Pats have him.
The second question is where the Pats rate him and what his projected value and developmental course would be. I wouldn't expect any DE/OLB conversion to come in and start right away. I wouldn't expect any pick at #23 to come in and start right away. If those are your standards for the #23 pick, then don't use it on Barwin. But don't use it on Butler, Beatty, Gilbert, Tyson Jackson, Everette Brown and probably every other player. Mayo was the exception for the Pats, not the rule. He came in at a position of extreme need and was a particularly quick study.
I rate Barwin as a 1st rounder. If the Pats do, I would argue that they will have to take him at 23 or at best 34 to get him. If they don't, then they should be happy that he will push someone else down to them.
Again, I happen to be partial about Barwin, but I try to be impartial about his draft stock and what others think of him. Obviously, just like this board, some teams will fall in love with him and others will be leery of him.