So... How much does the kit cost? Have you seen one installed in person, on an M3? Are you running a stock M42? Are you planning on installing this kit yourself or with the help of a professional body shop? Do you currently have a "whale tail" spoiler installed?
Basically it sounds like a horrible idea unless you 1) have experience with body work 2) are very serious about entering your car for competition racing 3) have tons of money/time to waste 4) are not an idiot.
This may seem like a harsh, out-of-my-ass response, but ask yourself seriously: Why are you considering this rather extreme modification? If you want to spend a bunch of money to get more out of your M42, consider an engine package from Metric Mechanics or sourcing a Downing-Atlanta Supercharger kit. Carbon Fiber has its proper place but would just be stupid on anything but the most extreme/dedicated track & race car.