I assume that the space lost to fuselage insulation and fittings are relatively constant across manufacturers (though some will claim an advantage), so extra inches in overall width are extra inches given to the cabin.

Thus, 10 extra inches on a 777 configured in 10 abreast seating is one extra inch per seat, given the same aisle width. That seems not insignificant to me. Then again, perhaps 10 abreast won’t be a realistic option for the 350 due to the smaller space.

Of course there is an aerodynamic cost to the wider fuselage, which some U.S. airlines have found to their detriment. I believe that JetBlue actually made the news a few years ago due to the large number of fuel stops their A320’s had to make with their transons (JFK-West Coast). Meanwhile, competitors narrower 737’s were making the flight non-stop. The alternative is to incur a payload penalty and leave people behind to facilitate an attempt at a non-stop.