Dude, I can deadlift over 400 right now anytime I walk up to the bar and I'm definitely not the athlete GSP is or I am severely deluded in a negative sense about my athleticism.
That was my point. I literally deadlifted on monday and tried to do a "lighter" session, I pulled 365 like it was a piece of paper and I pulled 415 pretty fast, after doing ~15 reps, then just did BJJ last night and probably will be going to a class tonight.
My point is that I'm not some special athlete nor have I ever seriously focused on powerlifting, been on a program, focused on technique of lifts, took steroids/PEDs, nor am I a ~huge person. Again...I used to pull 3 plates+ easily at 135-140 lbs bodyweight as a 16-17 year old, and right now I'm probably 190-200 roughly and I can walk up to a bar and pull 400+ easily, that's with training BJJ and having even less of a focus on lifting than I did prior.
If GSP focused his training on powerlifting, even while still doing grappling/striking work as a secondary focus, he could EASILY deadlift over 400 in weeks, if he doesn't already...which I bet he could.
We already know GSP fell in love for Oly lifting for awhile, and the picture above has him high pulling, or probably snatching 225 it looks like. Even if we assumed that was his max lift, snatching 225 is pretty tough and definitely translates to being able to pull 400+ off the floor most likely.
He is seen benching 225 here too, I don't think he's using maxmial weights for that either considering the whole camp focus lifting wise was explosive speed spectrum stuff, plyo and jump squats with a load, etc.
Yes you've never seen anything because GSP himself believed weightlifting was "retarded" to put it crudely when he was younger in his career. But he did do PL stuff and olympic lifting stuff later on, don't think he's the type of guy to brag about numbers or want to boost his ego with them as opposed to someone like Jones who went full powerlifter and tried to get max PRs at one point.
This part is just my speculation but I guarantee it's accurate enough: GSP could probably deadlift 500-600, squat 400+, and bench ~275-300+ within minimal time, a few months at most, on a PL program. Those might even be conservative numbers if he's free to bulk and/or use PEDs obviously which again is an open question regarding GSP.