I think I may have been the first to suggest a 450cc split, but after reading some of the other suggestions, I think it should be bigger. I bet the KLR/DR/XR 650s, LC4's, BMW 650's, Husky 610,630,650's plus the other random 600+cc bikes make up about half of the posts. 600cc is too big because most 600's are actually 598ish and the popular Husky 610 is 577cc.
That coupled with the true deffinition of a thumper being 500cc and up makes me think it should be 500cc. The problem is that while an old out of production Japanese XR/XT500 would belong in the upper group, the increasingly popular Euro sub 550cc bikes being built right now are more based off of 450 competetion machines. We have the older KTM RFS 520,525,530 and newer 501 which are all 510's, Husky 510 (501cc) and several others that do not belong with the heavyweights that are very popular.
I'm going to throw out a 550cc split for thumpers and seperating the two strokes, makes sense as well.
Or how about doing it by weight? no, never mind!