5.1 vs. 7.1 vs. 6.1 All opinions welcome.

I'm wondering if I should add 2 extra channels, or just one, or stay at 5.1

I am using a Yamaha HTR-6060, rated at only 90 watts per channel, and am already using it t drive 3 Sierras up front, and a 170 pair of surrounds (plus a subwoofer)

My concern is that diluting power between more speakers will reduce the quality in general, and take away more than it gives.

It also seems to me that there is already a very good surround effect generated from my two surrounds (which are slightly above, to the sides, and slightly behind sweet spot, slightly tilted forward toward the theatre seating.
Will it really get better, or is 7.1/6.1 a bit of a gadget that deosn't add much?

Can 6.1 work at all?

My room is large -only half is used as home-theatre and production presentation area, and the rest is used as dining/client conference and office. What I like now is that from the office area, surround still sounds correct. It sounds like there is a big orchestra at the other end of the room when I engage Dolby PIIx music or listen to 5-channel - no wrong echoes. (I'm lucky because all surfaces are either rough wood coffers or irregular, very old brick, so no flutter echoes whatsoever - just pro-spec diffusion).

I'm concerned that adding two speakers will mess everything up.

I know this has been discussed to death, but you gusy seem liek a very smart bunch, and I'd love to hear what you think.