I think I have something that might work to make everybody happy:
We obviously have the two camps, one for LiPo and one for 4 cell. I say go 4 cell/13.5 (kind of a GT1 category) and have a LiPo/21.5 category (kind of a GT2 category) GT1 can be for the Mustangs, Vipers. and Corvettes (oh my) and GT2 you can have like Aston Martin DBR9, Ferrari F430 and cars like that.