You are asking for the fastest car at 500PP and 550PP.
There is a definite answer to this, and it's not based on any opinion, its facts. They are as follows:
Roadcar 500PP: Suzuki GSX-R/4 regardless of what track you're on.
Roadcar 550PP: Honda NSX-R prototype LM road car or Suzuki GSX-R/4 depending on what track you're on.
Racecar 500PP: Opera Performance S2000 or Toyota TS030 depending on what track you're on.
Racecar 550PP: Toyota TS030 or Chaparral 2J depending on what track you're on.
Sorry if I spoil the fun in finding it out yourself, but as you compare both roadcars and race cars, it'd be good to know what is faster, my guess is that at 500PP the roadcars still win given the same tyre, but at 550PP the racecars are faster. But this is something I have never tested or seen, so feel free to test it out!
*Note: One thing to mention is that as the PP gets lower, but the tyre grip stays high (RS), there are a few FF cars that gets seriously competetive. At 500PP the Suzuki GSX-R/4 should still have the advantage, but go lower than that and the likes of Honda Civic and Ford Focus RS will give it a run for its money.
Edit: Note2: Given super grippy tyres (RS), and superhigh speed tracks such as Sarthe with no chicanes, there are cars that can beat these, but under normal circumstances, these 5 cars are the best.