Runs fine yes, but the bitmain driver throws away what could be potential block solves on p2pool...
the problem is only for s7? or even s5 - s3....?
There is a discussion that contains all modified antminer drivers for p2pool?
.. but it certainly isn't the only one

.
Do you think it's just bad luck or other?
Antminer binaries:
http://ck.kolivas.org/apps/cgminer/antminer/Bad luck, rising diff, crap firmware, lack of hash power - all of these are contributory factors I'm afraid, although bad luck & high diff affects all pools, not just p2pool.