If you cannot return or exchange these pistons, and they are unacceptable as-is, then they are junk. You spend only time to work on junk pistons and whether you install them or not, you gain valuable experience before you're done.

What 97 SL320 is referring to is the factory's balancing method. It's a single cut to save time and money. If you look at the difference in thickness of the pin boss between that machined area and the portion toward the center of the piston, it appears substantial. I wouldn't be afraid to continue the factory operation. I would still try to identify where the differences are between pistons before metal removal.

Just out of curiousity, are the specifications you're citing for the maximum weight variation for the 1.8 or 2.0 liter engine? Are both engines subject to the same limits?

__________________
When life gives you lemmings... make lemmingade.