abrcrombe, are you talking about my coupe?
My thoughts are comparing the '06 Z4 M Coupe at 52K or 13k plus labor for Dinan S3 added to my '99 Z3 M Coupe.
The S3 package adds free flow exhaust, ( I already have a Tri-flow that sounds awsome and adds 5 or so hp) lightweight clutch, springs, shocks, anti-roll bars, camber plates, 3.38 limited slip.
I'm also thinking the Dinan supercharger is supposed to be reliable and very streetable.
I have never owned a supercharged car, one reason being reliablity.
I would do a lot more research before pulling the trigger, and it's a lot of money!
What options did you get on the M5 and how fast have you been able to drive her?