You'll get funny wear with oversize tires on the stock narrow rims. Certainly with a 120 rear. I wore out two consecutive Dunlop 120s on the back of my 550s in less than 5000 miles. They squared off and lost center tread very quickly, and this ruined the bike's neutral handling. While the 120's fit within the fenders, they are a marginal fit onto the 1.85 rim width. While I haven't tried 100's on the front, I would rather expect the same type of handling issues would develop as the front tire "squared off" with use.
The narrow rim size changes the desired side will angle on the rim for a tire designed to go on a wider rim. This weaknens tread stabilty, centered on the rim. To compensate, higher tire pressures are required. This wears the crown of the tread faster leading to the tire "squaring off". They OS tire handle well when new, while they still have that new crown. But, the closer you get to replacement time, the worse the bike's handling becomes, forcing the rider to change riding habits to compensate. You REALLY notice it when you get a new tire that restores handling. It's a false euphoria, though, and lost as riding time increases.
I've gone back to the standard size tires for the 550; 3.25 Front and 4.00 rear. (Can't find a 3.75 tire I like.) They can still be ordered from manufacturer, even if the local tire Hawk doesn't stock them.
I've been happy with the Metzlers, and they've been wearing evenly, so far. There are still other good brands that make standard size tires, though they are dwindling as more people, put "modern" tire on these old bikes.
Cheers,
