It's always great to have more comfortable seating, but experience with Mazda Miata seats taught me that there are few other seats that will fit comfortably in a Spit/GT6. The main problem is the seat height. The Spit seats are situated virtally 1/4 inch off the floor pans.
Before you install other seats, please be sure to measure the height of the seat cushion from the floor pan. Compare this height to your stock Spit/GT6 seat. If it's higher than the stock seat, you may find yourself craning your neck to view stop lights!
This is very important if you're taller than about 5'9" or so, and of course, of paramount importance if the convertible top is pulled up!
I found the Miata seats to simply be too high for my likes. At 6'2", there's no way I can adjust the Miata seat for any real comfort, and that's after re-drilling all new mounting holes AND cutting the rear seat rails about 2 inches. Legroom and headroom are compromised, and there isn't a lot of room in the cockpit in the first place.
FYI, I hope this is helpful.