Ok, so I just built up/renovated a fat bike to see how it goes. 400 pounds [money: the sign for 'pounds' has disappeared off my keyboard, it gives me # instead] and I have a bike as smooth and flawless as you like.
I'm enjoying it so much I think I'll upgrade to carbon, and have seen a good deal on a frame that, like my current bike, is limited to 4 inch tyres, and a 170mm rear axle. [Most bikes now allow for 5 inch with a 197mm axle]
So, for Scottish touring, including winter work on the hills, is it worth paying a lot more for a bang up to date frame/bike that can take wider tyres?