In 1984, my second year of club roadracing, I was riding my RZ350 at Firebird. Some guy showed up one weekend on a late-70's Suzuki RG500 GP bike. Steel tube frame, spoked wheels, square-4 engine, kind of like Barry Sheene's old bikes.
In practice, this guy came by me exiting the chicane on the front straight, and that thing looked like a friggin' BULLET, literally shooting away from me in a cloud of blue smoke.
It looked so small and compact, and damn, was it fast.
I don't know who the guy was, but I don't think he was anyone famous. He just managed to get ahold of that bike somehow. I think he had problems with the bike that weekend, though. I don't remember him actually running the races.
That's been my only experience with a real GP bike. I wish I could get a chance to ride one some day, but it's not looking good at this point...