Ford is producing the FR500s for racing, and they are being campaigned in the Koni Challenge as well as other series (iirc). The biggest problem with Mustangs in general for road course is the suspension geometry. It's really tough to get a car to turn when the front and rear roll centers are as goofed up as they are in Mustangs. The cars can be made to handle, but it's usually pretty spendy.