I have a very old, first generation FFR and use it on the street and on the local tracks here in AZ. One of the guys in our club (Pro Auto Sports) drives a V8 powered Miata with 400hp. While it is much faster than an other Miatas on the track, it can't run with Cobras or Corvettes. Problem is the little tires can't hook up. Additional weight really screws up the handling too. My car is close to 400 HP and I can take him pretty much anywhere on the track, be it straights, brake zone or in the corners.
If you want to have a cobra...get a cobra. If you really want a fast Miata, put a supercharger on it. (My dad did that with his 99 and it is a real hoot to drive) If you still insist on a V8, there is a company in San Diego that can help you. Check this out.
www.monstermiata.com