p2pool with merged mining for 24hrs and bitcoin only for 24 hrs...numbers were the same, but maybe too short timeframe for real comparison. Why does merged mining suck? Doesn't the miner just hash what it's given?
Merged mining increases longpolling frequency due to multiple chains experiencing changes, especially when the difficulty of one of those chains is significantly lower. The negative effect on your mining is a bit of lost work per change. It would actually potentially be a lot more negative it weren't for longpolling.