The LP gnar1ta$ was refering to (5 per share) had nothing to do w/ merge mining. p2pool uses larger shares. 1 p2pool share ~= 150 difficulty 1 shares. It also builds a share chain to continually recalculate reward split and roughly every 10 seconds a new share will be found by the network and thus an LP.
Right, so if my work is being discarded roughly every 10 seconds due to an LP, shouldn't my scantime roughly match that to not waste so much work?
Exactly the opposite. If you're already forced to throw out work every 10 seconds, the last thing you want to do is throw out any more work than necessary.