I haven't finished measuring everything, but it's looking like 2 or 3mm on each side of my 530 chain. I'll have a better number later.
But, my question in this context was why the requirement of a 180 in the back if you put newer USDs up front? I'm just trying to understand the requirements so I don't miss something basic later. Why not put a radial tire that fits on the stock rim and a newer wheel/tire/forks up front? You can stuff up to a 150 on the stock GS1100 rear with minor mods to the brake stay arm no problem, after all...
I think you are reading too much into this by calling it a requirement :|. I would say that 95-98% of USD conversions have a 180/55-17 on the rear with a 17x5.5" wheel. If they dont then they are simply running a 17x4.5" wheel which is certainly more rare and avoids some clearance issues; most there that come to mind are on GS700/750E (83-85) and not GS1100E's.
Most of your cornering performance comes from that rear tire, so having a low fat profile is the objective.
As for the stock rim, you would be ill advised to put anything bigger than a 130 on a 2.75" wheel and until you get to a 150 you can even find a radial tire. All of the 140's are bias tires.
