I thought the same thing about the 8 benefitting from adjustable side bolsters, supergoat.

The question is about the seats, not the interior space on the two cars... most know that the Miata is smaller with less leg room for the passenger and driver.

I like the seats in the 8 over the Miata partially because they are taller, have decent styling and fit me well (in leather, the cloth seats are less padded).

 

I haven't owned a Miata, but I used to own a Triumph Spitfire. The seats were great, but on a long drive (7-8 hours) it was hell. Since my legs were almost straight out, most of my weight was onto the base of my spine. I'd get out of the car and the pain in my tailbone was excruciating. The seats in the 8 give you a lot more room to move your buns around to different positions and seat adjustments. Last time I made a 3 hour drive it was very comfortable.

 

Wife and I had a 91 BRG Miata (leather seats) and traded it in on a 2005 RX-8 GT. The seats are a lot more comfortable in the RX but then I have not sat in a later model MX-5. The GT has power adjustable drivers seat with lumbar support and is very roomy but provides plenty of support for cornering. The passenger seat is manual but also can be adjusted for comfort. Just put a little over 1800 miles on a week long vacation and never had any problems being comfortable.