The SkyActiv has been used in the Mazda3 for a couple of years now (2.0L and now 2.5L), and so far they seem to be just as reliable as the engines that preceded them. You can run just about anything at the track. The key is to make sure you have adequate cooling, and that the brakes will hold up to heavy use without fading.
From my research...
You can get the typical CAI and Exhaust upgrades to unlock a few more ponies, but so far there have been no significant gains. The engines are already tuned to run a @13:1 or @14:1 compression ratio from the factory. I really doubt we will see an aftermarket turbo or supercharger for these motors. They are built for fuel economy. If/When Mazda puts out a SkyActiv turbo, and tuners can get a look at how they are building/tuning it, we may see some progress.
The pictures of the new Miata show a higher redline than the Mazda3, and MPGs have never been a priority. So I am guessing the output of the SkyActiv in the MX-5 will be tuned for performance.