You know, if you have the unquenchable thirst for speed, a Miata can be made to be fast and reliable for not a lot of money. Take your Spec Miata, add a $3-4k Supercharger kit (or up to about $5k turbo kit... or DIY something for half that) and suddenly it's scary fast. It's not "spec" any more, but it's fast.

S2K parts are definitely not cheap. Have you considered open wheel? Something like a Stalker would make a really fun dedicated track car.