For auto-x, you want a supercharger. You want smooth, constant power. Trying to corner on a short course with a turbo means most of the time it wouldn't be fully spooled and when it does kick in, the last thing you want to be doing is coming through a corner. SC all the way. What the heck class are you going to run with a FI miata with all those mods? If you use the SCCA class rulings, you may want to check to make sure you're not modding yourself into the bottom of a very high-powered class. I know with sciroccos/rabbits, once you add forced induction, you have to be prepared to make the car pretty much a serious race machine to stand a chance, since it puts you in the same class as 'vettes and porsches.
As for Vdubs, yeah I'm into them hardcore. The raddo was pretty nice, basically VW's next attempt at a sports car after the scirocco. However, the supercharger in them was crap and the VR6's are silly expensive for parts. They're definitely not what I would call reliable and bulletproof, but then most all VWs fit that bill. I own an 84 rabbit GTI and a highly modified 81 scirocco street/road track car.