Fixing this issue will likely require a hard fork of the p2pool share chain.
It looks like that's what will be needed then, as it seems that most (80%) of the hash rate has finally come to an agreement with Core to increase the blocksize:
http://www.coindesk.com/bitcoin-miners-back-proposed-timeline-for-2017-network-hard-fork/So I'd recommend v0.12 (classic or core, up to your taste) to every p2pool node out there. I use
blockmaxsize=930000
in the bitcoin.conf on my node and don't have any problems with orphan rate or tx errors.
According to forrestv on github:
I wouldn't recommend for you to change that value. Depending on where the problem is, it could either work or split you off from the P2Pool network.
...which I think is what happened to me & I had to re-download a new sharechain. If it works for you, that's great, but I'll take forrestv's advice & keep it at the default setting of 750000 until/if the problem has been fixed.