Have a look at a few other threads on this. It keeps coming up. I run 100/90 x 19 Avon Roadriders on WM4 (2.5") Morad alloy rims. I think I'm the only one. The wheels are somewhat lighter than stock with the same tyres. Avon recommend this size rim for this tyre. They also say that 2.75" and 2.15" rims are suitable alternatives. They also say that this tyre is also suitable for the standard 1.85" rim, but with a more mushroom profile and tread at the edges that you just can't use. The Roadriders are not a traditional type tyre, but a modern one. They are made, as per Avons website, to suit the 2.5" rim. This is the way I went and would have no hesitation in doing it on any future Nortons I get my hands on. 18 inch rims and tyres do not make Commandos handle better. They are for "looks". They can, indeed, amplify certain Norton handling traits which are less than desirable. Wide tyres on narrow rims can make steering feel heavier than it should. The 100/90 Roadrider on a 19 inch 2.5" rim puts more rubber on the road at any lean angle than a 120 section tyre on a 2.15" rim.
The sharp steering and excellent stability have been commented on by others who have ridden my 850.
As my rear tyre wears out, I fit a new one to the front and fit the front tyre on the rear. The advantages are that the handling doesn't deteriorate as the front wears and I only buy one tyre at a time. You cannot do this with mismatched tyres front and rear.
By the time you have fitted a wide tyre and rim to have any more meaningful contact patch on the road, the combo won't fit in the standard swingarm. My combo only just fits after a couple of slight mods. One is to turn the right rear shock mount bolt around so the head is on the inside of the swingarm otherwise the tyre won't fit. The other is to use button headed cap screws on the front mudguard mount attached to the forks or the front tyre won't fit.
Go ahead, do it. you'll like it.