A Monster Miata can be built for just about the same price as the Jackson Super Charger, and maybe even the Flying Miata turbo set up.
So it you want only speed, think about that route.
As for me, I don't want to upset the balance that the Miata already has built in. So I go turbo.
You can install a turbo for less then the Flying Miata, but you had better do some research first. It takes more then a bolt on header, turbo charger and exhaust to make a turbo system work right.