RS's are the 'track' spec - they have very little in the way of luxury stuff and they don't have the Active Yaw Control etc. Very much a drivers car. Shorter gear ratios too as i remember...

GSR's are the 'comfort' spec - they have all the luxury bits and the AYC etc. Still a nice drive, but heavier and taller geared.

FQ's are 'spruced up' with very average and exceedingly expensive parts.

MR's are the daddies I would advise anyone to get an MR as the bottom end will do 400lb/ft and a bit as opposed to throwing a rod at 350lb/ft, the turbo is bigger and the cams have an extra 1mm of lift...

Hope that helps...