Solid reliable numbers should just be a matter of time, I mean the wind tunnel IS getting built and even if it doesn't do full airflow visualization it will still give us solid, (hopefully) repeatable numbers for downforce and drag.
Mass production, however, would be a little bit sketchier. Many Mini-Z bodies don't have a very good place to mount aerodynamic devices, this particular 911 GT1 is a very special case -- but it's not a very popular racing body. I'd probably have to figure out diffusers for the Ferrari F430GT, 360GTC, Mosler MT900, McLaren F1 LM, Lexus SC430 GT500, etc. etc. as those are MUCH more popular and more people would buy a diffuser for them. Cutting the actual diffuser itself would be easy, it's just a flat piece of plastic cut to a pattern, which the user then bends to shape and adheres to the shell.
*Shrug* possible, but I'd need to buy or borrow a lot of shells.
