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.