But overall, I'm starting to come up to conclusion that p2pool rejects ~20% of its hashrate overall?
mainnet p2pool will reject the last share mined in each block interval. For Litecoin, a share is mined roughly every 30 seconds and a block is mined roughly every 150 seconds, so this causes 30/150 = 20% of shares to get orphaned. P2pool will report "punishing share" and "block-stale detected!" when this happens. This effect happens to everybody almost equally, so it does not cause imbalance. The mining that is done in the last 30 seconds of each block interval can still be used to mine an actual block and pay all p2pool users, so it's not lost hashrate; it's merely unaccounted hashrate. However, if you are a large enough miner, you can mine another share on top of that when it happens to you and prevent your hashing from being unaccounted like everyone else's, so this mechanism *is* a centralization/selfish mining risk.
I eliminated this mechanism in jtoomimnet:
https://github.com/jtoomim/p2pool/commit/b57a4ff93e58c0702aa2481c164517e7290c8d43https://bitcointalk.org/index.php?topic=18313.msg18580559#msg18580559