I see a lot of modified Miatas and they are excellent track cars.
They are extremely light and can be modified (Forced Induction) to have about 300+ WHP , Limited slip differential and rear wheel drive.
Because they are so light they dont go through tires/brakes as often.
I realize this is a BMW forum but that is probably one of the cheapest/best bang for the buck track cars you can get. You can pick one up used for under 10k and spend another 7 to 10 k to make it an amazing track car.
I always see these Miatas at every track event I go to.
2012 E92-M3 Space Grey on Fox Red, DCT
2011 B8 S4 S-Tronic , Black on magma red