Your dead on! I have a note book and I write down everything after each race. Gears, tranny, engine, RPM's, CHT, EGT, ect.
A good tach will only help you with this. You sound serious. So stay away from those cheep dial tachs, those only "look cool" and have no real function. Go with a digatron, you just need to decide how many functions and features you want.
Paul