stiffer springs give more RPM
lighter more torque
Yeah. really
As AdrianM pretty much said it about the C2 and them not liking being over-geared. I have spent enough time with them to know what works over here in the UK on our outdoor tracks on rubber tyres. Like I'm certainly going to tell him a set up that screws his motor so that it needs zapping every run!! He has already asked about how hot they get so must be concerned.