I think that for a public road-driven car, most of the CSL's character is due to the gorgeous soundtrack, so by installing a nice sounding intake on the Z4M you'll be most of the way there.
I've driven a friends CSL for a day once, it was on Nitron suspension, AP brakes and shitty Chinese budget tires. The sound was glorious, the ride was very firm (as expected) and the feel from the car was absolutely HORRIBLE. My Z4M on stock suspension but good tires had much better steering feel and much more connection between the front and rear axles, I don't know how to explain it but the CSL felt as if there was about a second's lag between when you asked the front axle to do something and then the rear axle following. I am confident though that that is due to the bad tires on the M3. His brakes defintiely felt better than my own Stoptech kit, the intake noise was addictive, the straight-line speed was maybe better but i honestly couldn't tell from behind the wheel (he does beat me in highway drag races though

), the looks were awesome but the Z4's are better and I know for a fact that his car is much faster than mine on the same track (he has dedicated track tires). That said after driving it i've not really felt the need that i need to drive it again and if my car had a similar soundtrack it'd be almost as good if not better overall.
Again, the cheap tires are probably at fault there and probably with a decent set the car would feel much better but that's the way i experienced it anyway...
For me, i've no use for 4 seats in my sporty car, my car's plenty fast already, looks amazing, sounds amazing and i'm not so fussed about lap-times. My Z4M is just right for me at the moment.