John Plut, check the thread titled "free HP for 124..." in the performance paddock. It will explain most everything there.
Jackd, I removed mine and yes it did make a noticeable difference. You must run 91+ octane fuel which it sounds as though most people do. I simply put fresh electrical tape around the holes where the resistor was to keep debris out.
Try removing it, putting in your ashtray just in case, and see what you think. Mine runs like I feel it should have without the resistor! Evidently it makes up for the lesser octane fuel found in the States.
01 S55 AMG Silver
01 CLK55 AMG Silver