With plastic as with wood, faster speeds result in less splintering/cracking but with plastic there's a window where when reached you create fricton that causes melting. That being said, your only going through say 1/4" at the most? how big is the hole? If its 3/8 or smaller i'd said at least 200 rpm or higher. The thing you want to avoid is the sharp cutting edge of the bit to catch as it breaks through. Faster is better on plastic.