Just a word from a happy Corbin user. My corbin solved my versys seat problem completely. The Corbin is firm, but it does break in some. It provides excellent support for me, and I can ride all day in comfort. YMMV.
As for the pillion, I don't sit there and I don't take passengers. It does look kind of small. You will need to take a look.
As for the backrest, I had one on another bike, and they are okay. Pretty solidly mounted. But a top case with pad will definitely be better.
+1 to the Twisted Throttle suggestion. I have the SW-Motech rack setup with a Givi V46 top case (and E360 side cases). The one or two times I had a passenger, the case provided good support. The SW rack is very very sturdy (more-so than the GIVI IMHO). -- Hank.