Well, I've gone and done it. This is partly the product of some conversations with Bimmer, so if this sucks, blame him. If it rocks, of course, it's entirely my idea.

I've always liked spec racing. The idea that it's all down to the driver/setup is appealing to me. I've also noticed a couple of things in Forza that strike me as a bit odd.
1. The cars are VERY light. It seems all the grip machines are incredibly light. A mustang with full WR in-game comes to about 2400 lbs. The Mustang used in the Mustang Challenge is roughly 3400 lbs, and the review of the car in GRM was pretty glowing.
2. We're "stuck" at certain PI points. If you want to run an open room, you pretty much have to stay at the top of the letter classes.
3. There are WAY too many high-end parts on the low-end cars. It's hard to imagine an entry-level racer using all the carbon fiber and exotic techniques described in the flavor text of the platform upgrades. I can't imagine a cheap MK2 club racer using a custom gearbox, carbon fiber driveshaft, and all the stuff mentioned in weight reduction. It happens, sure, but not at the lower levels, and it's certainly not the price of entry that it becomes in Forza.
Solution? Only the most popular spec series in the United States! What follows is a Spec Miata, built as close to SCCA regulations as Forza Motorsport will allow. I didn't really pay any attention to PI. As a result, these cars are NOT for open-room use. In fact, they're kind of rubbish in an open room. If you're looking for a setup, I'm currently running a 53.5% WB and 75% SWR, and it's fairly dialed in. Note that this build includes a stock transmission, clutch, flywheel, driveshaft, and differential. These cars are being given away in the online racing section, but here's the build.
Power: Bone Stock. (Spec Miatas even run with a restrictor plate, putting the front-running cars at about 115hp)
Platform: Sport brakes (non-adjustable), Race springs and ARB's, Sport WR.
Tires: Stock Width Toyo DOT's with Enkei rims (or any other rims you want-SM specifies rims must weigh at least 13 lbs, but I figured this could just be left "free" in the game)
Body: Mazdaspeed front bumper and spoiler. (In the future, I may either go back to stock body parts or make non-forza body parts all legal, as you're generally trading off weight vs. drag in all cases, and the downforce doesn't seem to be all that important.
Decal: Yeah, I put a vaguely SCCA-style number decal on the ones I built. It's not required, but I think it'll add a neat look to photos of the races.
Well, that's it. Yes, all of it. This is, as best I can figure, what cheap, entry-level racing looks like when translated into Forza. If anyone has any concerns, critiques, or suggestions, they're welcome, but keep in mind that the point isn't to make a fast car. We're all (theoretically) equally fast in these cars. The real-world idea of spec racing is to make the checkbook less important and the driver more important. The checkbook is less of an issue in forza (even poor guys like me are sitting on a couple million CR), but this should provide a nice change of pace. Please critique, whether by eyeball, by building your own, or just signing up for a free one in the "Spec Miata Giveaway" thread.