You misunderstood. QoS allows bandwidth for other traffic but for it to work you must have some headroom, if you need all of your available bandwidth for other traffics you can't use p2pool, if you only need what's above 256-512kbps in both directions then you can fit p2pool and bitcoind in with various levels of efficiency.
See my prior post. What's your 12 day comparison? I realize it's probably too short, but I'd like to see. We can compare again in 18 days.
You don't have to ask here, the information is already public. Go to p2pool.info and you'll have data for a theoretical 100% efficient p2pool node.
Given that you mentioned a cube I suppose you have an ASICMiner cube. Unless they have a different miner implementation than the blades you are probably out of luck on p2pool. IIRC the blade miner could only do getwork and poorly: it didn't work well on several pools including p2pool because of their implementation. If this didn't change you are probably better off letting the beast mine on whatever pools you could find that worked fine with it.