I've never run an N6, but I've cut in lots of places I couldn't go any faster, but I seldom have never cut anywhere that I couldn't go slower. so as long as you are happy with the straw spread, I say go bigger with the header!
All depends on how heavy a crop and the job the swather operator makes.... If its a nice even swath I think the combine will handle it, If lumpy forget it. We use a 25 ft swather in heavy canola and I think its a better match for our N 6.