YES!!! I have read a lot of reviews of receivers in the past. I had a Yamaha at teh time. They tested it with all 5 channels running and it was like 75 w/channel then with just the mains, it dramatically jumped to like 110w/channel. I think that perhaps the best thing to do is to buy a fairly cheap 2 channel amp for the mains,and use the receiver for the others.
like someone already said, it may not be a big differnce, buy either is going from 75 to 110 watts in any system, but perhaps when you are watching a 5.1 movie and you need more headroom, it might pay off.
Yes, you'll get more power, but why? Mass market receivers tend to use the same power supply for all 5 (or 6,7) built-in amps. Leave some channels unused, and viola, more power....power that may become noticable during peaks in sound like during explosions or gunshots. Fun stuff!
I have a HK 5800 power amp where the amp tested out in a review at 110 wpc pushing just 3 channels instead of the normal 5 channels (where it is rated for 80 wpc). Alot of this will have to do with the quality of the power supply in your receiver. Your mileage will vary.
Expect cheaper 5 channel receivers to not even provide the wattage they claim when pushing 5 channels. I have seen some rated at 100wpc x 5 only manage 31wpc x 5 (see latest Sound & Vision on the Sony STR-DB1070). That's why I prefer separates - the power supply is dedicated to only the amp.
The real issue however is that the difference in loudness between 80wpc and 110wpc is minimal. If you could get a 3db gain (80wpc --> 160wpc), well that would be a noticable difference. 1 or 2db as stated by Patrick is not going to buy that much in terms of loudness. Your largest gain is going to be the additional wattage from the second receiver/amp.