While I haven't hit the track yet, I would agree that 20psi does seem low. Although when I was riding DH mountain bikes, I would run 20psi and rip through the rock gardens without issue. Most other riders would run at least 25psi on the same mountain. Not the same, but all this to say, what most would think to be too low a pressure, might be right for you.
I run 26psi on the street, but that said, among other factors, it also depends on which tire you're using, what the ambient temp/weather is doing, riding style, track conditions, etc....Just something to think about.