Pretty sure the 2J can be dropped to 600pp, it will thrash pretty much anything in its pp range normally, so I would suspect it is the same at 600.
At 500pp I find my quickest car is an FD RX7 such as the premium Spirit R. The NSX Type R is generally the car that seems to win around there tough, at least at more cornering-focussed tracks.
I never really race at 400pp, occasionaly 350/450 but not 400, but suspect an MX5 would be a decent choice for that sort of pp, at least that would be my choice.