I'm assuming the 2.5 Gen B650 has the same frame and linkage as the 2nd Gen.
If so, a very cheap upgrade is to find a 2nd Gen B12 shock. I got mine from Mongo on this board (thanks again Mongo) and it is better than the stock 600. A stiffer spring and a little more damping (but not a lot).
You can look at putting in a 'busa shock, like the 2nd gen 1200 guys, but it will bring up the rear an inch or more. IIRC the 2.5 Gen 650 went back to a 120/70 front (the 2nd gen 600 has a 120/60) which raises the front about 3/8", so a busa shock may be workable.
Otherwise there's lots of aftermarket stuff, but it's pricey...