I have consistently higher stales than the pool. Today I mined my HD8650 for 8 hours non-overclocked (so at ~210Mhz instead of ~250Mhz) and that did not improve it any. On average p2pool reports ~16%+-16 while the pool is usually about 8% (today it even reported 23%+-23% !)
Is there anything I can do about this?
What reject rate does your rpc miner show? Are you running NTP (is your clock correct)?
The big -/+ means you don't really have much data for the estimate. Those ranges mean that in fact your true stale rate might actually be pretty low.
Because you're a bit behind when you start its usually to get a couple stales right away that makes the number start out poor but then it improves with time. It's not unusual for me to see stales at 20% shortly after startup but by efficiency is usually at 100% by the next morning or so. (converges a bit faster if you have more hash power though)