Don't think of it as competing with miners on the same node, you are completing (and collaborating) with the whole pool.

Also, node efficiency is a poor indicator on any node with more then 1 miner. A single miner with a high latency can bring node efficiency way down, while your miners may be mining very efficiently.

Look at your latency to the node, the node getBlockTemplate latency, and your stale/DOA shares to get a feel for your particular efficiency...

After a few days of mining you can look up your miner in our "active miners" list for p2pool. That estimated hashrate, if higher then expected, means you are mining efficiently/currently lucky. If it is lower then expected you are mining inefficiently or in a streak of bad luck. (this only works after you have been mining for the length of the share chain, i.e. 3 days)

http://minefast.coincadence.com/p2pool-stats.php