Barwin would be a good fit in GB. It comes down to cash. Is he looking to strike it big? If he's looking for a good chance to get a ring, GB is the place. Plus, he needs to be in a 3/4, which fit him better than a 4/3.
We play 2-4-5 80% of the time. Now, if his trouble with the 4-3 was lining up in a 3 point stance that's one thing. But if he's another Fackrell who won't be able to set the edge in the 2-4-5 I'd be hesitant to pay him starter $.
2-4-5 is simply 3-4 Nickel. The concepts and techniques are 3-4 concepts. We just play in a "sub package" much more than the base.
This is just wrong. The 2-4-5 looks a hell of a lot more like a 4-3 defense than 3-4.

I've never been able to figure out how to insert an image properly, but this link shows what our 2-4-5 formation looks like. (Edit: Well, look here. I did it!) Quite often we'll see the Elephant on the strong side line up in a 3 point stance. The technique and responsibilities of an EDGE player in a 2-4-5 are essentially the same of 4-3 DE.
http://insidethepylon.com/film-study/film-study-nfl/defense-film-study-nfl/2014/11/29/packers-passing-preview-defending-high-low-opposite/
OK
This is NICKEL 3-4 DEFENSE.
The elephant position is somewhat unique to our defense, but defenses arent "cookie cutter". They dont say " Ok guys, we are a 3-4 team so at all times we need 3 defensive linemen, 4 linebackers and 4 defensive backs on the field at all times." They play in different roles.
What you are looking at here to over simplify it. are 3 down linemen, 3 LBs by position and 5 DBs (which is where NICKEL comes from). Now you see the DB at the bottom of the screen (29, I believe). He is a DB by position, but in this set he is functioning as a LB. We put him in that spot in order to get faster more athletic guys on the field and have better match ups.
Now we also run a version of this with our dime group. Where both guys on the edge are standing up, 2 down linemen. Then one LB in the middle and 6 DBs on the field. Two of those DBs are in the slot and essentially functioning as LBs. This is a a little more like a 4-3, but again these arent cookie cutter defenses.
In Barwin's case, he would almost exclusively be asked to play with his hand up and out in space. Not with his hand down on the line. Perry and Jones if re-signed will probably play the elephant role this year. They are more of that DL/OLB Hybrid while Barwin and Clay are more of the true OLB role.