Miatas are great track cars but they have serious aero issues. If you start getting in the 300+whp range they can easily achieve 140+mph on some tracks and things start to get pretty dicey up there, and the short wheelbase doesn't help.
For a high-powered track car, I'd rather have an E36, and I used to race a Miata. But if you MUST have a V8 and you don't want to buy 18" tires, just get a Miata and drop a small-block ford or GM LSx engine in it. You can do that (and make it reliable for the track with an upgraded diff, bigger brakes, and cooling) for less than what most new M3 owners spend on option packages. A cage which ties the firewall to the rear of the car is a must at that point because the chassis will twist itself in two.