At 600PP I'd go for the downforce.
I've been testing with several cars and there seems to be a breaking point.
Example: Car has 400PP, 150BHP and 40 downforce (lets say 40 represents the total amount of df)
Car runs ring in 8:00:000
Same car, 400PP, 250BHP, 20 downforce: car runs 7:30:000
Just because the difference in BHP is so huge (percentage wise)
Now we take a 600PP car.
600BHP, 120 downforce. Car runs 6:00:000
Same car, 700BHP, 80 downforce, car runs 6:10:000
So it's a matter of: how much BHP do you already have, whats the weight of the car and how much downforce can you generate in total?
I'd rather loose some BHP if I have like 700, than loose downforce.
Only when a car has really low BHP I'd go for the extra power instead of the downforce.
On this track, with 600PP cars, I'd definately go for the downforce.
You have to realise though where you have to gain the time.
If you have a high downforce car and get stuck behind some low downforce opponent, you HAVE to pass him in the corners because he will totally annihilate you on the straights.
IF you wanna have fun online do this:
Make server, allow only cars from the online garage.
The only 2 cars allowed: Ferrari F40 and Nismo 380RS Super Leggera.
The F40 has speed, the Nismo has grip.
It's really fun to drive with 5 F40's and 5 Nismo's on the Ring.
Try it.