As new as the Powerboost is it's apparent that very few people here have heard of it, let alone towed with one. I would suspect that the extra weight of the batteries and the electric motor is only going to hurt you MPG wise when towing.
Hybrid is great for around town stop and go driving. When towing it's likely just in purely gas engine mode. I must assume you have some kind of display that tells you when you are in electric/hybrid/gas mode. Have you watched it while towing? How often is it in each mode during a typical trip?