For 600PP events, I've been trying to use tuned road cars like the AI does, where as for the WTC 700 events, I use a tuned Gr.4 car on Racing Softs - specifically the free Mustang Gr.4 that I won via the missions, with a supercharger thrown in it, as it reaches over 690 PP with those parts. I do something similar for the WTC 800 events, but with the Corvette Gr.3 that I also won from the missions.
So back to the original question, regarding WTC 600 events, I've been doing quite well with a modded R32 GT-R on Sports Softs. With a mid-range RPM turbo, anti-lag, a torque vectoring controller, and pretty much every other possible part equipped - and with some power limiting and ballast - I'm able to get real close to 600PP. And it takes off like a rocket on straights, thanks to the aforementioned turbocharger.
As a nice bonus, my R32 is also fantastic for drag, mostly due to the torque vectoring controller allowing me to set the torque to 50:50, which feels amazing for drag. Of course, for drag, you'll want to switch to a high-RPM turbo. This is also true for if you're on a broad oval like SSRX or Daytona Speedway, and both large ovals and drag could be even better with the rare Ultra High-RPM turbo. Otherwise, I'd recommend a mid-RPM turbo to use in general.
I'd experiment with multiple road cars on Sports Softs, either those you've won from the menus/missions/licenses/roulette, or those you've bought from Brand Central/Legendary Cars/Used Cars, and see which one fits your preference(s) best. I'd also pick a car somewhat similar to one that the AI is seen using, either in the actual race, or in the "typical opponents" section of the menu. I would imagine that some cars would be a lot more viable now, since the most recent patch helped to mitigate the exaggerated wheelspin/oversteer of many rear wheel drive cars.
