I see that the Model 7100 has specs of 7 x 165 watts @ 4 ohms, 20 - 20kHz, 0.05% THD, All Channels Driven.

I have a 5.1 setup with 4 ohm speakers.

My question is:

Since I would be using this amp only for a 5.1 setup (so that two amp channels would not be used), then would I actually have (theoretically) even more than 165 watts @ 4 ohms, on the five channels used?

I believe that you would still only have the 165 but I will let the experts respond on that end. I am curious...which speakers do you have? I have the Polk LSIs which are also 4 Ohms and I found them to work better with the 770.( I used to have the 7100) They just seemed to open up more with the extra power and now I have more headroom for those volatile movie soundtracks.

In a 5.1 setup, you would have less load on the power supply, which might give you a bit more capacity to react to rapid changes in demand, but you would still have 165W per channel.

As for the speaker impedance (4 ohms), a better gauge of whether or not you would need more power is the efficiency. The Polk LSi15's are 88dB efficient, which is a very low efficiency. My Paradigm Reference speakers are around 91dB efficient, a more "middle of the road" efficiency, and have been very happy with 165W (using an older Model 750, 165W @ 8ohms). Soundhound has some horn speakers with an efficiency well in excess of 100dB, and he drives them with amps that are rated at somewhere around 5W or so - and has power to spare.

Howdy,
I'm running 5 maggies(4 ohm,low eff. speakers) off the 7100.The 7100 seems to have plenty of headroom,and Ive yet to hear any sort of clipping.

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