Well, let me tell you what I know. Being one of the drivers of the above listed 1.8l that finished behind the supercup 996 at the last Summit 12 hour, I feel I can speak w/ some experience.
The car we drive is a '95 that started out as a motorola cup car, then went SM and is now ITS. The rational was that in a 12 or 24 hour race, SM is just a long sprint, there is nothing to use as competative advantage. In ITS the lack of abuse on parts is the Miata's edge. This keeps it out of the pit lane more than the other cars. In sprints this theory doesn't work (obviously), the race needs to be somewhere in the 6 hours or longer range (I think).
The car won't make the 2200# min weight legally, at near zero fuel and a 155# driver we're at 2305. I've looked at everything on that car for weight and you might find another 25#.
I'd be impressed if you could make 150 crank HP legally. We've done the .040 over and headers and messed w/ air cleaners (the only thing on the intake you can change due to its layout) and I don't see it getting there. The guy building the motors hasn't found it all, but I am confident he has moved the big rocks. Now that I've said that, he'll probably prove me wrong (hope he'll try at least). The good Sunbelt motors that I have seen in SM on the dyno are in the 120 range (w/o restrictor). Maybe someone else can say more about this.
We've run Khumos, Toyos and Hoosiers on it. The Hoosiers did not do huge things for it, the Toyos in the long run were easier to drive therefore yeilding more consistant lap times.
I would disagree w/ the comment that there are limited gear choices, I was under the impression you can get most any gear set 4:10 through 4:50.
I've run it in sprints and found the same thing w/ the enduro qualifiers. It doesn't have what the RX'7, 325's and 240z's have for overall lap times.
The factory support is awesome. In comparison to my own Nissan's, Mazda comp is amazing. The car owner will send me an email telling me that parts are on their way, and the parts are on my doorstep before I get to read my email.
It is also a hugely fun car to drive.
Hope this helps.
Matt