It sounds like your client and pool node are working properly, and there is no parameter which will make the p2pool node set the worker difficulty to the share difficulty.
The p2pool node will take care of adjusting the difficulty for your mining client so that it is not submitting work too often. It's not a bad thing for your mining client to be submitting shares regularly to the pool node, even if they are at a lower difficulty than what is required to get a p2pool share. Regular traffic between the client and the pool node allows you to verify that everything is working properly and it allows the p2pool node to measure statistics about the client.
From the the wiki:
Q: Why does my miner say it has found a lot of shares but p2pool say I have only found a few?!
A: The real P2Pool difficulty is hundreds of times higher than on normal pools, but p2pool essentially lies to your miner and tells it to work on relatively easy shares so that it submits shares every few seconds instead of every few hours. P2Pool then ignores any submitted shares that don't match the real share difficulty. By doing this, P2Pool can more accurately report your local hash rate and you can see if you are having problems with too many stale shares quickly