Efficiency was close to zero with an Avalon mining stratum on p2pool.
Odd and worrying: you are not the only one having tried mining on p2pool with Avalons. I don't follow it closely (forums are not really the best for bug reports, I have to work from memory and I'm on painkillers right now...) but I know people have managed to bring Avalons at least in the 50-75% efficiency range right from the beginning and made progress from this point (maybe not with stratum).
I suspect there's at least one Avalon user on P2Pool for quite some time now which uses it without apparent problems given his payouts (185Kip6odGYs4eSHD6DYsWVDJBg2DNLfiV since 2013-09-04 with an effective ~65GH/s hashrate).
He mines to multiple pools. I think two are on the same machine? Not sure..
But his efficiency rating would be higher if he sent me a msg and told me his IPs, unblocked them (right now the nodes I know he uses don't allow incoming connections) for my IP and let me connect
