Regarding this:
There appears to be an issue with p2pool producing lots of orphaned shares if the blocksize is greater than about 750 kB. This is caused by the limit on the number of transactions per share being too low.
https://github.com/p2pool/p2pool/issues/274I've been in contact with forrestv on github, if anyone else who is having this problem can send him their log file he would appreciate it. There is a "tentative" fix available on git but I'm unable to test it atm, so if anyone can try it & provide feedback, that will also help forrestv out.