Adding a shim may be necessary to use 15% on a regular basis. I would have a couple on hand just in case.
There are dimensional differences in some of the ST models made in China that were also made in Italy. Example is the G51. The bore of a Chinese G51 is slightly smaller than the Italian G51, so an Italian ring will either not fit or bind if used in a Chinese model. I'm sure there are others, but that's be one example Frank Bowman made me aware of when I picked up rings for my G51.