I don't think you will see almost any increase in power by using onl 5 of the 7 channels. The only common part to the power supply is a 7 channel toroid transformer. Each channel does its own AC->DC voltage conversion and each channel has its own caps to store the energy. I spose there may be a tiny interactiion between channels due to flux density changes in the transformer. I'm thinking its really small...
I definitely concur with one of the previous posts. The different in speaker efficiency is going to be the real factor.
Mike