Search this thread and you'll find a wealth of info about Miata seats in Spits/GT6s, and other seating options as well.
The Miata seats in a Spit/GT6 render a higher seated position for the driver, reducing headroom. Thus, if you're taller than approximately 5'11" or so (I'm 6'2"), it's uncomfortable. The Miata seat is also wider which cramps the right leg against the gearbox tunnel cover. In addition, Miata seats change the angle at which your foot rests in the pedal box, cramping the accelerator pedal foot (uncomfortable on longer trips). Finally, the increased height of a Miata seat may cause taller drivers to crane their neck in order so see stoplights through the top of the windscreen (yes, it's true!).
Having said all that, some folks love the Miata seats for Spits/GT6s. I do not. I installed them in my MKII GT6 (after much wrangling, measuring, cutting, and welding on the seat rails, etc.), and hated them. They tend to make the already tiny cockpit even more crowded. Jerked them right out of the car and reupholstered the stock (high back) seats. Much better.
Further, I don't think the Miata seats look right in the car, but that's just me. A Triumph isn't a Mazda . . .
The stock seats may be made far more comfy and firmer by using modern, non-foam materials in place of the stock seat foams (which provide little or no real cushioning). I purchased firmer seating materials from a local upholstery shop and it made the stock seat significantly more comfortable.
The key to the fit of the stock seats is they sit VERY low in the floor pan, a <span style="text-decoration: underline">must</span> for any seat in these cars: this is critical if you (or your passengers) are tall or large.
Don't install Miata seats in your car if you're relatively tall, or if you're a large person, or if you intend to accept passengers who are. Very uncomfy.
That's my tuppence worth, YMMV.