I wear 7.5 boots and ride a mid-wide board most of the time (258 mm) and a somewhat narrower board (245 mm) sometimes so maybe I have some insight...
Advantages for the extra width:
-No heel or toe drag ever
-extra float in powder
-Additional stability on flat landings
-slightly slower edge to edge on hardpack
-difficult to find bindings that center properly
It's definitely slightly worse on hardpack than riding something closer to my foot size, but of all the things I do on the mountain, hardpack is the last thing I give the slightest shit about. For me the advantages in float, stability, and the lack of heel/toe drag in the steeps far outweighs losing some quick carving ability. It is a huge bitch to find bindings that I can center myself with, most simply don't have the necessary adjustments to keep me balanced, but such is life. All that said, the whole board width thing just isn't as big a deal as most people make it out to be, I can hold and pressure my edges just fine without any toe or heel overhang. It may not be ideal if all you roll out on is groomers, but it makes up for it by rocking extra hard in the pow.