First, I didn't test all cars. Second, most/all of these tested cars have absolute maxed out performance, which means junkman parts included. Third, I tended to fiddle only with the steering, handling, and turbo/supercharger sliders in performance settings. I usually set handling and steering to +1/+2 (depends on car). I'm not sure what exactly the turbo does, but I try to make it so that the gauge is in the "positive" region as much as possible. I try to judge these without nitrous usage too. Here's my opinion on the ones you can use:
Porsche Carrera GT: fastest all around car in the game. Great acceleration. Top 3 in turning ability (with stability) department if you tune the slider to +1 or +2. You do lose out on some acceleration, but it's generally worth it. I set turbo to +4/+5. Since I used this car so much, I also set ride height to +1; seemed to help somewhat on the world loop sprint race.
Lotus Elise: Best handling car there is without tuning. The only car I have been okay with tuning handling/steering and aerodynamics to -1 (not that this helps). I believe this also has -5 turbo. I'll check again later.
Ford GT: A slightly worse Carrera. Works fairly well; this is either -5 or +3 on its supercharger.
Corvette C6: A slightly worse Ford GT. +3 I believe on the turbo.
Porsche Cayman S: For the time it's available, it can outdo a lot of the other cars you have access to.
BMW M3 GTR: Not a car you normally buy, but I feel it's worth mentioning this car is 2nd place in the handling department. That's on default settings too (not that you can change those). Sticky, good turn radius, and easy to handle, it's surprising that the game lists its handling stat as so low. It does take a bit to get to max turn radius though, since you can't adjust it. Other downsides are it's slower in a straight line than almost any fully upgraded car you can make, and its nitrous is horrendously weak. Still really good for a car with no turbo/supercharger; the best in the game if that's taken in consideration. This is what made me suspicious of the car's in-game stats in the first place.
Mazda RX7: It had a good balance of handling, speed, and acceleration. Not terrible, but nothing makes it stand out either.
Subaru WRX StI: Handling was horrid last I checked. However, it has great low end acceleration. I think it also is like a tank in pursuits.
Lamborghini (Gallardo I think): Above average acceleration. Average turn radius. Stable and relatively quick off the line.
Mercedes SLR Mclaren: This thing can't turn. At all. The only thing it may possibly be good at is throwing cars around. Maybe accelerating to 100 MPH too.
Astin Martin DB9: This also can't turn at all. Even worse than the Mclaren. On long courses, about the same overall time as a vastly stat-inferior porsche cayman S.
Other Mercedes in the game: All handle like crap. I also did the longest tollbooth time trial (challenge 67/ a race in Razor's list of races) in a CLK 500 versus the Porsche Cayman S. This had outright better stats compared to the Porsche, and was about 12 seconds slower than the Porsche. That's not an exaggeration. (Note: CLK 500 had a supercharger at minus 5 for the most boost I think).
Cadillacs: Average acceleration and speed, horrid handling. from what I recall.
Mazda RX8: It did not build up speed well. Handling was at best average.
I'll probably come back to edit/add to this later.