Sorry for the late response, I have been travelling the last two weeks.
As you probably remember I have a 7.1 system with two P17b's as sides and two P27s's as rears. The reason why my system is that way was really more space driven than anything else. The recommendations I got were the same as you are getting, use P27s's for both sides and rears. I know that you have seen my pictures and there you can see that I have windows on one side and open space on the other side of my seating area - so no place to hang the P27s's - I therefore ended up with using the P17b's as sides.
If I had more than 5-6 feet of space behind the seating area I might actually have gone for P17b's as rears as well, and this is simply because I think the design is gorgeous (stupid I know). But as I only have about 1 foot between the couch and the rear wall I use the P27s's.
What can I say about the sound? To me it is awesome. First of all I will say that moving from a 5.1 to a "proper" 7.1 - by proper I mean using sides and rears in addition to the fronts and center, not front wides or front highs - is amazing. The full immersion sound field created with 7.1 is just in a different leage than 5.1 if you ask me - especially now that a number of blurays are 7.1 or 6.1 (watched all the 6 Star Wars movies the other week - simply stunning).
I cannot say that I think the P27s's disperse the sound too much - but then again I do sit quite close to them so my situation may not be all that compareable and relevant for you. My recommendation though is to try the P27s's if they are available - I do not think that you will be disappointed
Good luck and let us know what you think!!