Speaking from practical experience, Street Racing is NOT considered Racing.
My experience comes directly from actual Road Racing in Professional as well as Club Racing levels. We have a variety of cars in our Team ranging from 2 Turbocharged Mazda RX7's in Speedvision trim, a 944 TurboS in PRO Racing trim, a 944NA in Clubracing ITS trim and a Mazda RX2 in HSR B-Sedan TransAm trim. Oh yeah, also a Landspeed Record car.
The PRO series includes a wide variety of cars ranging from Vipers, 911 GT2's and GT3R's, 944 Turbos, Saleen Mustangs, Supercharged Mustangs, World Challenge Corvettes, Supercharged NSX's, Ferrari's, and much more. What I can tell you is that the most dominant cars are the Vipers, Porsche GT2's and the "one off type Porsche 914's". The 944 Turbos tend to run towards the front of the pack along with the Corvettes and the NSX's. The Ferrari's tend to break early in the race and the Mustangs tend to either be last or break early. In the Los Angeles Grand Prix this year, the fastest Mustang was driven by Bruce (Tom?) Griggs of Griggs Racing. PRO rules are based on the past finishing records of each vehicle and so the Mustangs were placed in a slower class. The 700+HP Griggs Mustang was placed in P3 Class along with 250Hp Mitsubishi's and stock 944 Turbos. It did win in P3 but was also the ONLY Mustang to finish (4 started). Amongst racers, it has been an ongoing joke that the Mustangs are very unreliable and require significant chassis mods to make them handle. The Griggs Racing K-subframe is evident in almost every serious Mustang Race Car. Even then, one of the other Mustangs left the circuit backwards when it's subframe broke for no apparent reason and sheared off part of the drive shaft. In other forms of Professional Racing, the Saleen/Allen Speedlab Team left the American LeMans Series after 2 years of disappointment in the GTS ranks, a class where the Vipers and Porsches rules. Even my friend who is one of the engineers at Saleen/Allen Speedlab told me: "The Mustang chassis just is not competitive, it's a poor geometry design". That coming from the Ford Factory Racing Team! They had to finally build a prototype car, the Saleen S7 (built by Ray Mallock of Ford Europe) in order to be competitive against a 10 year old Ruf Porsche. Of course the S7 is a total monocoque Carbon Fiber design with mid-engine layout, not an ounce of Mustang in it.
As far as Club Racing is concerned, our 180hp 1973 Mazda RX2 regularly beats the stock Mustang GT's on the track. A few friends of mine race Mustangs and are dedicated Mustang nuts. They even own a huge company that builds engines for racing Mustangs (RPM Engines). We've had an ongoing bet on who will win the championship. So far, we've beat them two years in a row. Their 500-600hp American Sedan (AS Championship) Mustangs have either broken or can't keep up in the turns. This is against our 350hp Mazdas. Even they finally admitted that the Mustangs are not the ideal performance engine. A good friend of mine recently bought a Mustang GT40 and modified it for clubracing. After chasing problems for two years, he finally threw in the towel. Anybody want to buy his Mustang Race Car? He'll gladly trade it for a 944 Turbo half the horsepower!
This is not to insult Mustang owners. Obviously, many of my friends race Mustangs and we still hang out together despite their poor choice of cars. Seriously, I'm sure the Mustang is a fine car for drag racing or for the "Traffic Light Jockey". I even considered buying a convertible for my wife as her daily driver (we got a Lexus SC400 instead). But as a Real Race Car, it's design has proven to be a "Dead Horse" (pardon the analogy).
For references on my facts, you can visit these websites:
PRO Racing Series:
www.PRO-Racing.com
(you can also get the RPM Engines link there)
Our team:
www.RealRide.com/racing
(you can read our race reports)