(Edit: Everything you see here is completely outdated since the Wild Run patch from 17th November 2015. Well to be honest it was probably already outdated in March/April 2015

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This question has bothered me quite some time. I decided I need an answer and went on to test this.
So I bought all 14 circuit spec cars and tuned them up to 1299. All of them have the same lvl 50 platinum bonus parts equipped: 4 Grip, 4 Brake and 3 Top Speed bonus parts. These parts were pretty much the only parts I have, I didn't spend the time to grind for 11 Grip or Acceleration parts.
Now I needed a track where I could get consistent times. I tried the "Bottom of the Ninth" mission first, but it was too long and the traffic was too random. So I chose "Circuit Breakers" instead. There is no traffic, the AI cars aren't interfering and the track is pretty smooth... well it better should be, it's Laguna Seca.
I did between 10 and 20 attempts for each car within the last 2 weeks, until I was satisfied or couldn't improve the time anymore. Overall I spent well over 10 hours on this track. These are my results:
Keep in mind that I'm driving on
hardcore handling, so this list will look different with full assists. I'm also not a machine - some runs were better than others, sometimes I did minor mistakes. Some of these times could still be 1-2 seconds faster with more luck or more optimization. But my results definitely show the potential of the cars and it's completely clear that the SLS and the Ford GT are two of the best circuit cars. Also the Alfa and the Hotrod are two of the worst cars (no doubt).
I added the Grip, Accel, Top Speed and Brakes values so you can compare them. There is a tendency that cars with high grip are faster, but the individual handling is sometimes very different, even when the grip values might be similar.
Edit #1: I forgot to mention that I'm driving with an automatic transmission. Also, with some of the cars I didn't use nitrous a single time, because I only lost time with it. Examples are Lambo Murcielago, Hotrod, Alfa - these cars would be faster with full assists. Some of the other cars also don't make much use of the nitrous - so consider this a hardcore ranking. It would be interesting to test the cars again without nitrous... but I don't want to see this track anymore
Edit #2: The Point scores are varying so much because of the "lead percentage" bonus points. Cars with alot of wheelspin lose some points at the start.
Finally I have some video footage to give you an impression how these laps looked like. This is my best lap with the Mercedes SLS, which is also my best time overall and the current world record on Circuit Breakers:
I'm planning to test street, perf and dirt spec, too. But I'm still currently busy collecting all the cars. Let me know what you think
