If only 85% of my accepted shares are useful, is the 15% that aren't causing a reduction in my possible payout over a pool that accepts 95% but then takes it's 2% cut?
p2pool shares and other pools shares are not the same, p2pool will have 10-30% stale shares and still be healthy without hurting your payouts.
It's been stated many times in this thread, but to be clear: the 15% rejects you are seeing are not wasted work and can solve a block. It's also possible that it does not reduce your payout.
p2pool works differently than any other pool. Rather than look at your DOA/STALE/ORPHAN rates, you have to compare "Efficiency" rates (on your console output and web interface). If you have 20% p2pool stales, you may have 100% efficiency - which is the number that counts.
So again, when comparing pools, don't look at p2pool stale rate, look at p2pool "Efficiency" (because p2pool stales are not the same as any other pool stales).