Ye the switch is worth the extra cash really, mainly if you are planning on adding other mods to the car.
I've not had it on the RR yet, i'm currently waiting for Revo to get back to me as i'm not totally convinced its running stage 2+. The car has been running a few different specs lately so im a bit lost with it at the moment! I was expecting some problems with getting the power down (which you will from a standing start sprint) from a rolling start the wheels will scrabble a bit in 2nd and 3rd, nothing too serious. I couldn't make any ground on a '03' 996 Carrera 4S today, they are 316hp and weigh 1470kg so its hard to say.
I was running aggresive settings too so in theory the car should be producing around 340bhp.