Worst mod was a quick shifter stick on my S1 Elise which was quickly removed. Best mod was the Lotus supercharger kit on my NA S2 Exige. It was pretty cheap (at the time) and transformed the car, I was never happy that the cam change could kick in on long sweeper curves like Corum. Supercharging made it much more linear and easier to drive and the noise…
Reminds me of back when my car was NA. Best mod was the ECU reflash for the lower cam change. Made it completely linear for track use. I know some people preferred the drama of the cam change, but definitely not desirable on track.