In my original post I agree with this. The only logical situation for the Bills to pick him up is to trade down. Like you said though, there is going to be a plethora of "better" DE's available with the 11th pick, that it's hard to see us trading down a few picks to pick Barwin up unless our personnel folks are extremely high on him.
If he somehow falls through the pats and we're looking at him in the 2nd, he would be a steal. While he probably won't be an immediate starter at OLB, I still think that is his best NFL position and is certainly a position of need for us.