I use mine as a coat hanger.

You realize abortions are legal. You don't need to use a coat hanger anymore! :facepalm:

This is identical to the brace sold on ebay for 30. I ran it for a while and didnt notice a difference, then someone gave me a MSM brace and the change was noticeable. Probably cause the ebay brace you posted is a flimsy pos

Does anyone know off hand which NB braces don't fit under the NA hood?

The NB Tein front brace won't fit the NA without some trimming of the hood substructure.

I use an OEM Mazda front strut brace from a Mk2.5 Sport but on my mk2. I figure they designed it and put it there for a reason so it must offer a minimal positive in terms of engineering? Saying that, I can't 'feel' a difference.

It's worth noting that the Mazda bar is a straight cross-over bar bolted at either side of tall mounts on the strut body not like those ebay bars that have two bends in the cross-over bar. I'd imagine they would just flex if put under any stress whereas the Mazda bar being straight, would offer zero flex strut-to-strut.

Will one of those Ebay POS ones still work for a MC brace?

I just built my own STB with MC brace with stuff I had lying around at home. I noticed a change in stiffness with the brace installed and removed, both with brakes and front end stiffness. https://www.miataturbo.net/suspension-brakes-drivetrain-49/diy-strut-brace-master-cylinder-brace-44167/

can you sell me one for the rear?

These are for making sweet MC braces as far as a track miata is concerend. On a street car I heard they help with the 65 shimmy.

I bought a fancy Carbing 3 point brace because I found it cheap and my wallet was weighing me down.

I can grab both ends and flex it with my own hands (this from someone who struggles to bench 150lb).

The amateur ME in me cannot see how it's stiff enough to make a difference as the manner in which the cross tube is connected to the shock towers acts a huge lever - the load transfer is not very direct.

-Zach