What's wrong with this car?
I'm sure you have a bunch of your own witty answers. Things like, "It's a Miata", "It can't fit my purse inside while I'm driving", etc. Really, there's only three things wrong with any Miata generation. First, they are wrongfully made fun of. Secondly, the Miatas aren't very fast. Thirdly, Miatas also have some noticeable oversteer. However, this snap oversteer is fairly controllable compared to other cars (like Corvettes. Sorry Bobbo, but it's true). The Miata's problems end there.

Now, the Miata is extremely light. They're very well regarded and well known for their handling, low cost, and reliability. However, the latter doesn't factor into Forza (if it did, the 458 would burst into flames upon sight, etc), but the other two do. The Miata has extraordinary handling qualities, and it won't put a dent in your pocket. It's the perfect low cost, low class short track car stock. However, the snap oversteer and power need rectifying.

Mazda Powaaaaaar
Now, what I did first was improve on the Miata's amazing handling. No sense in trying to make a speed demon out of the Miata. It's a short track car, and should stay that way. I upgraded the rear tire width to the max, which vastly helped the snap oversteer problem. It's still there, but greatly reduced. Also, I added sport tire compound to further reduce the oversteer, and help with cornering. With a full race suspension, full weight reduction, race anti roll bars, brakes and driveline, the Miata gets even better handling-and the oversteer is almost completely gone. The rear end will kick out sometimes, but compared to stock, it's an extremely drastic reduction.

To solve the Miata's speed/power problem, I fully upgraded the exhaust and added a sport air filter. This allows the Miata to sucessfully keep up on short tracks, corner quickly, and give it a fighting chance on anything straight. A sport gear box, and clutch help the car shift quicker, furthering its acceleration. The Miata makes a legendary short track car. That's what I kept it as.

Online
Well, I'm not the best driver, but still, this tune absolutely dominates on short tracks. Imprezas, S5s, and the like, just can not corner like the Miata whatsoever. Practically nothing in C class can. I even took it out on Suzuka, and it can keep pace with the LB cars. I would expect midfield finishes or higher on short tracks, but leave this car in your garage when dealing with anything like Catalunya, Sebring, et cetera.