This is a funny thread.
Ive had both cars, road and track, and my M3 has ended up as a 1200kg full on race car (new mods just being finished as we speak

)
There is a really really simple answer to the original question, the real world difference between an M3 and CSL, ie, not laps of the ring, not which is the lightest,not which is worth the most,just if you jump in and drive both cars whats the difference. Simple simple answer.
The airbox.
Nuff said