How much do you think GSP Benchpresses?

Probably not that much, the bench press not really a big concern for martial artists. The use of the pectorals muscles are fairly limited in martial arts as the only time it is really used is in the follow through of a punch (see for your self, feel your pec with one hand and throw a punch with the other. it doesn't really contract until the latter portion of the punch), and pectoral muscle tends to be quite large if emphasized which is just extra, mostly useless muscle which makes cutting weight that much harder. It has pretty much nothing to do with getting tossed around, the muscles primarily responsible for that include mostly every muscle except for your pecs. But my guess is that he Could bench anywhere from 320-380, not an insane amount but enough to get that follow through on those punches.