HDPE cuts really well with standard bits. They have to be very sharp though. If your bowl bit has any chips or is dull, the result won't (literally) won't be pretty. HDPE cuts with similar feeds and speeds (in my experience) to medium hardness woods. I suggest taking off 1/16" first to experiment, varying the F&S to get the best cut with your bit. No matter what you do, there are going to be machine marks. HDPE doesn't sand well either.
ShopBot Details:
PRS Alpha 96x60x12
4hp Spindle
12" indexer
Aspire
Rhino