There is a world of difference in the design of the 12A & THE 13B rotors
Ok, I'll take your word for it. I've never looked that close

There is also a world of difference between a 220 rwhp 13B & a 130 rwhp 12A in terms of trap speed
I recall the Miata's running 13" wheels in EP, even though they are allowed 15x7's (and winning the runoffs). I would have to assume that you can generate equal or greater exits speeds with the 13's. (might be application specific and only work on Miata's). In other words you can generate the same loads on the hub.

Leave the shortage of 12A parts for a seperate discussion.
Why? It is becoming an issue with the old british cars currently running G&H prod. Adding another car with dwindeling parts availability doesn't seem to be a good long term solution to car counts.

I'm not trying to rain on anybodies parade. In the grand scheme of things it would benefit me. It just adds another class my car could be run in and keep some value. The car I'm currently building is a 13b EP car, it would just be an engine swap to go GP.

Good luck,
E