I lose a lot of races trying out new sleepers. It's not too fun winning with a popular car.
The answers so far didn't really answer the question full on. HSC is well known 500-550pp rabbit, while the Evora is slightly less potent than the Elise, but popular nonetheless.
These are some of my non popular cars. I won't list any too popular cars (M3s and NSXs etc.):
The older Esprit models (non premium) seem to serve me well. The Opel Speedster is an OK touge car with the MR2 GT-S. Any Skyline is good for beginners alongside the Legacy. FF racers should try the Prelude (any), Volvo C30, FTO GP, and any Golf models. The 86 gang are excellent for 500pp with the Alfa 8C.
Many cars require proper tunes to compete, which cars ahead may need. I can recommend the Pagani Zonda C12 (NSX comparable at 550) and the Countach Anniversary (400hp at 500pp). Most exotics are bad if majorly detuned for 500pp but should be good for 550pp, if you know how to drive or tune. Vintage muscles like Charger and Cuda, and pickups are more for style points, while the premium ones do need tunes for the sleeper effect to kick in. I have a Art Morrison Corvette for 550pp and it's no stranger at winning.
This is just a bit of what i know.