Over the last couple of years, a few of the Chicago racers (including Rob King) were racing with what they called "murdered out" flat black cars, and they looked pretty cool when decaled and numbered up. Kevin Kane ran a flat army green NSX at the USVTA Scale Nats this year that looked really cool.
I have been tinkering with the idea of replicating the Chevrolet Z06X and Z28X concept car matte pearl white on a USGT body, too...
Traditionally, Lexan paints up from the inside for durability and shine. If you want flatness or a matte finish, it's certainly an option to paint the outside, but it's a lot more difficult to get a consistent finish when painting that way.
Not to mention that many paints WILL wipe off if contacted with denatured alcohol, WD40, Naptha, tire sauce, etc. if you are cleaning tire scuffs off the body with any type of fluids at any point.