The seats are mostly for comfort on the track. Better support in corners but less comfort on the street..
The passenger seat is the old Sparco Sprint used in my previous NA.
The new seat is a Sparco Sprint V with side mounts, better support in the bottom and compatible with 6-points harness. Its also a bit more flashy with Sparco logos on the shoulder padding.
I originally tried to fit a Sparco Roadster seat but that was not possible with the stock rails witout cutting the fiberglass shell.

I have always thought that the NB has the seats mounted way too high. It feels like you are riding on top of the car instead of in the car like the NA After som test fitting of the new seat I discovered that there has to be some changes from NA to NB regarding the mounting points for the seats since the Sparco seat feels much higher in the MSM than in my old NA. I'm not able to move the seat under the roll bar which was possible in the NA with the same bar.
It might be the rails that are higher and beefier since the MSM mounts the seatbelt in the rail and not trough the transmission tunnels as in older Miatas.
To fix this I will make new ultra low unadjustable mount from "flat iron" on the driver side. I never use the rails anyway other than when removing or fitting the seats.
For longer trips I'll mount the stock seats and rail. I'm not driving 3000Km to Nürburgring with the bucket seats.

Some pics of the install:

I used lots of time making offset plates to fit the boltpattern of the sparco seats with the stock rail. After much trial and error the best method is to mod the rail like in the pictures. This way it is possible to use offset plates in the front and make new holes at the center of the rail since the rail is now paralell with the sparco bolt pattern.

Here are the seats mounted in the car. I'm not satisfied with the ride height. The stock rails and the offset plates are too high. I think Bride sells seat rails that will fit and that are smaller.