Stuart, unless there are sound reasons(no, not intended)for thinking that the 3803 doesn't have adequate power for your M60s(highly unlikely), there's no reason why they would sound "sweeter" with M200s. Unused power is simply that; unused.
As to rear speakers, you may receive some benefit. You could choose to have your 3803 play the M2s for the surround channels on DVD-A/SACD if you prefer the more direct sound for those formats. Although there is no such thing as a 7.1 format, in addition to the relatively small amount of 6 channel material, processing such as DD EX or Neo:6 can be used to expand 2 and/or 5 channel formats to give the rear speakers some work to do.
