as far as costs go, even if I budgeted 15k for for an ls1 swap na miata (doing the swap myself) with brakes, tires, suspension..etc. It really wouldnt be cost effective when compared to the cost of consumables.
Say for the m3 5k for a bbk, 2k for wheels/tires, that leaves 8k for consumables for the m3. Thats a lot of tires/pads/rotors. Where the idea of having a track car comes in nice is if I ever were to put the m3 into a wall (shudder). Not only would a track miata be safer (roll cage), but it would be less expensive to buy a new shell and move the existing parts to a new car.
I could build an ls1 miata vs 47's ls6. that would make it a bit tamer, but still a ton of fun, and quite a bit cheaper. I also like the idea of building it myself. Seems that if your going to track a vehicle, you should be able to inspect/fix it yourself.
Got to admit, 15k for a monster at the track seems like a good idea