5% fee is a little steep IMHO.
Have you costed or tried to run a reliable service? Hint: this is not running on some oversubscribed $10/month VPS, it's running on dedicated blade hardware in a professional data centre.
Having said that, the admin fee will be be reviewed regularly and reduced over time as the pool size increases. So far this month the 5% fee has covered about 2.5% of the monthly hosting fees.
As an alternative I was thinking of modifying P2Pool to create 10 times as many shares with a 10th of the difficulty but otherwise identical, would anybody be interested in running this ?
You would need to run a separate P2Pool network to do that, which means until you will suffer from basically the same variance as solo mining until you are able to build up the volume. The main P2Pool network with 7TH/s has an average of 2 blocks a day, how many miners would you need to get even close to that?