While it seems obvious, one of the first things you should do when you make more power then factory intended is decide which spark plugs to use. Not just any over the counter spark plugs will do, and most likely if you walk in to an auto parts store and ask for spark plugs for a 1990 Mazda Miata, they'll give you ticking time bombs(jk). I'm only partially joking though because in the perfect storm, they can and will cause preignition or worse, detonation. If that happens you'll be cracking lands like its going out style… Long story short, don't overlook the spark plugs!!!
For turbo Miata's making over 170 hp the most successful plug seems to be the NGK- 4644 (BKR7E) gapped at .025" for stock coils and anywhere from .030-.050 for aftermarket coils (typical toyota cops .032-.035)
The heat range of your spark plug plays a huge role in the success and well being of your engine, check out the ngk link below for more on that.
Also see the MT thread link below for confirmation of the spark plug posted above.