I saw that Tremec has came out with a 5 speed TKX transmission that appears to be a reasonably priced choice for a V8 Miata build.
Currently (as of Nov. 12, 2020) it is listed at $2795
You can get a version with a shifter location to match the desirable 4th generation F-Body cars and it is listed as being suitable for 600 ft. lbs. of torque with the GM standard 26 spline shaft.
The article I read mentions that it has smooth shifting even at 7000+ rpm.
It also fits into a smaller transmission tunnel than the T56 and is lighter.

 

Hi BG,
Great post.
I saw this announcement as well. It looks like a great transmission and one that I would jump at if I was doing it all over again. I do like the T5z, its pretty small, its light smooth with a nice shift action but it feels a little "soft", and I know that with constant abuse and 425+hp it would likely let go.

I might look into this transmission in the future