512 objects are not really harmful to your FPS. But it all depends on what you use, and where you put them while in the driving path.
An example of low FPS is that city driving course with tons of walls. You see the walls cluttered in a bunch, and that is what can ruin video performance. The main goal (if you wanted to use a full 512) is to use objects that are smaller and less detailed, and not clutter them up so much. Use as much spacing and the fewest objects as you can without ruining the course design.
Cones are the way to go, and maybe a few signs as walls. I honestly do not like the red/white barriers because they are buggy and bad for performance if you use too many of them in one spot.
The simplest courses with a full 512 objects are the best. Cones, Poles, and maybe some tirestacks for apex of turns are your best choice. Chalk lines helping to guide people are useful as well. That is how I make my courses.