Honestly, start with the stock pistons and see what you think. If you feel you need more rapid suspension action I would then give it a look.

Me personally the RC Shox pistons were quite beneficial for really rough sections. They also worked fine on indoors carpet. I didn't see the need to change them out, just use the appropriate shock oil.

As for which pro drivers use them, don't, etc. I think Mantisworx can comment on some names directly who does, don't, etc. for his product line.