who sells these 11" friction rings w 8 bolts?!
 

I bought em used, I think they're Goodwin V3's, which are now out of production... ditto replacement rings.

https://www.miataturbo.net/attachmen...1&d=1409096025

I thought the V2's were out of production. The V3's are from DBA I think.

Pretty sure those are fastbrake. The iron that fastbrake uses is not compatible with all brand of brake pad according to them. Note sure what the offset is or if other rotor/hat combos line up. Maybe Brian would be willing to share that.

I get the occasional call for rings for some obscure Miata wilwood kit (there have been many). Thus, we offer our SPM 11" and 11.75" wilwood kits optionally without calipers.

I've been using them with XP8's... braking is excellent, but the rotors seem to be wearing fast.

What dimensions are needed to figure out if I can use other 2-piece rotors? Diameter, thickness and offset? Which way is offset measured? Inside surface of hat to outside surface of friction ring?

That is readily transferable. I'd make Goodwin your first call.

I would replace hat's and rotors with something more common. That older one off stuff is tough. I really liked the 949 directional rotors when I ran them. That's a standard kit with wilwood hats that will not have obsolete problems that the goodwin kit did down the road.

They are made by Racing Brake. For replacement friction rings you can go to Coleman Racing, send them the specs for your rings or just send the used rings, let them know what features you want, i.e. floating, gas slots, directional, cryo, etc... and they will set you up. FWIW, 949 sells a'la carte so it MAY be cheaper to get new rotor brackets and wilwood hats/friction rings if you opt for all of the bells and whistles on your replacement rings.