Hi,
I used to have PnP enabled....but recently I disabled it, even before I moved to 1.33NA with no ill effects.
I think the important thing for the 360's is to DHCP reserve by MAC each 360, and then in the 360's Network setups set it to automatic. I also have DNS relay enabled so I even set the DNS setting on the 360 to automatic and it picked up the DIR as the gateway.
I don't think I have anything else set special for the xbox's here and they all have easy access....