The best for 800 PP, well, for that it's ANY Gr.3 car since most if not all can't even reach 800pp and they would still be far faster than any other the AI.
But the overall best car for 800p for the WTC800 is the Chaparral 2J. Just add max ballast and take out HP from it until it reaches 800p. It ends up with about 495 HP and over 900kg, which makes it much faster than any Gr.3 car and it grips on slow corners like no other car in the game, while the other cars are doing hairpins at 70-80kph, the Chaparral does it at 100kph. I easily on the outside of any corner just overtake the AI casually.
You just have to be careful out of corner exits and high speed corners when you are doing them at high RPMs, at that point, just short shift it and you are good, the car tends to oversteer sometimes.
730pp Gr.3 however does seem to be a whole different ball game. I can win the races with the Gr.3 Corvette, however, the one in Daytona is really hard, the time for you to pit is just about when the leader pits, and for some reason, your pitstop takes longer than the others even when taking just the necessary amount of fuel. I went into the pits in that race just behind the leader (which was a yellow Nissan GT500, the Penzoil one) and I came out of the pits, 13 seconds behind it and with other cars in the way who pitted after me. Overtook the Nissan on the last lap, but this is something that shouldn't have happened.