I've raced mine in SS middleweight and heavy, SB middle and heavy, GP middle and unlimited, GTU, GTO, and F40. To be honest, no one is runing a true Superbike or Unlimited bike in the non SuperStock classes. Same guys in pretty much all the classes, just less of them in the SBK and GP. Still, it's mainly middleweight and it can get packed (especially at NJMP).
I found most guys are running 600s or 1000s, so Heavyweight is kind of moot. Still don't understand why there's a GP class, but hey, it's more racing for all of us
I prefer to stay out of MWSS and MWSBK because of the crowds, especially turn one! Just getting old I guess.