5S was the max cell count in the rules, which is heaps. 500W is totally doable on 3S, your just wasting money if ya wanna go HV
500W is 500W, If you get their through voltage or current there is no difference in performance. Going HV is certainly more expensive though
No battery capacity limit, but again, there is no gains to be had with a giant capacity pack, it'll just slow ya down around the corners.
Getting around the course cleanly is the best plan of attack, and will easily result in the best gains.