Until then, p2pool is actively DoS'ing the bitcoin network, by delaying transactions until a non-p2pool miner builds a block.
That is very harmful to the network.
No, it isn't. P2Pool is contributing to the security of the network by building on existing blocks.
As for delaying transactions, P2Pool running without including transactions is exactly the same as everyone using P2Pool just not mining at all. (Until the next difficulty change, at least.) Block generation is independent, so the average time to the next non-p2pool block is: 10 minutes / (1 - p2pool_hash_rate/total_hash_rate), which is currently equal to 10.01 minutes.