I doubt he does. He said he has a bad radiator and a bad thermostat? Why not add a larger capacity radiator and a new thermostat? I'm bewildered and think there are pieces missing from the story.

The very first question should have been "do you still have the factory radiator undertray and ducting?"

 

the radiator and t-stat have been replaced along time ago. i now have a 2core auto radiator. need to install an oil cooler like yesterday. the undertray is long gone and the PO cut off where the under tray connects to so putting one back will be difficult. it dosnt get near as hot but, when its hot and hummid itgets warmer than i like. and i just liked the look of cspences white one on miata turbo

 

Doing it for looks is just fine. If you want to cure an overheating problem like the one you mentioned, sealing the mouth opening in the bumper to the radiator is of primary importance. Secondarily, an undertray behind the radiator is known to make a notable difference. After that I highly recommend a 53mm Mishimoto or equivalent radiator as it will have better than twice the capacity of the auto trans radiator. Then I would consider a Kia waterneck reroute.

But even with a big 53mm radiator a Miata would possibly overheat without any ducting before and/or after it. It is just how the Miata was designed.