Interesting that a bunch of people on a website with same/similar cars can't agree on a set of rules. I'd hate the job, you could never keep everyone happy, especially in a transition period.
Having said that, here are Duncan's motorsport categories, hereby solving all problems.
2 categories - sports and touring cars. sports cars have 1 or 2 seats and can be open or closed. touring cars have at least 4 seats.
3 classes in each:
Standard: Only allow reliability mods like coolers/radiator. ECU and suspension free. Brakes free under standard size wheels. Street tyres only.
Improved: No aero. Engine swaps from the same manufacturer with the same number of cylinders. Suspension pickups can be moved slightly. Gearbox and diff internals free but not sequential. Wheels +1" wide and +1" taller than factory. Semi Slicks only.
Open: Aero. Cages. Spaceframes. Slicks. Sequential gears OK. 4wd/2wd/front engine/rear engine/whatever, go for it.
Each broken into capacity groups: 0-1600, 1601-3000, over 3000. Turbo, rotary or supercharger x2.
Not many classes, so not many hard choices. In total 2 x 3 x 3 = 18 groups to place within.