You'll have to call and talk to Chris in the performance dept at Carls about the pistons. I don't think that they'll sell you just pistons if you didn't buy the kit from them. Worth a try though.
The difference is also in the porting and the custom re-flash (re programming) of the ECU. Being in Bay City.... the flatlander HTG setup is probably best for you.
Carls has their 975 kits dialed in for Mountain use. Their kits are designed as a system with the pipe, head, porting, reeds, Clutchig and a custom ECU map.... fine tuned to work very well together!!