I, of course, vote for Flyin Miata / Flyin Protege. They have over a decade of turbo experience on the Miata. A 6 psi kit will give your car almost 170 wheel horsepower - check the dyno sheets at fmprotege.com.
They are working on a fuel management system that will allow 12 psi on stock internals. Should be ready very soon. Out of all the kits, FM has said they are confident they offer the most user friendly and reliable kit of the bunch. All of thier components, by the way, are of very high quality. Having visited the shop and watch them work, seeing how clean the operation is, I could not recommend anyone else.
The kit itself was designed by Corky Bell, who has been designing turbo getups for decades - not years, but DECADES. The FM kit does not require you to change anything else - not the clutch, no extra injectors needed, nothing. Plug and play.