I found another possible source for cheap and common injector shims. I was fixing the AC compressor on a car today and needed to a shim washer to set the air gap on the clutch plates. I grabbed my bag of McMaster-Carr shims and found they fit perfectly in the clutch drive. Thus, one might go the other way - use AC clutch shim washers in a M-B injector. These clutch shims should be fairly common. I don't know the increments, but most air gaps spec 15 - 30 mils, so they likely come in 5 mil increments or 0.13 mm, which increases pop pressure ~130 psi.

Another thing I'll add is recent experience with my 1984 300D. I found that 3 of the 5 injectors popped ~1600 psig (instead of 1950 psig), which means they were non-turbo injectors. I installed a set of 5 from my old 1985 turbo engine that failed, after testing that they all popped 1900-2000 psig. I haven't noticed a difference in idle or driving. I thus surmise that pop pressure is probably not extremely important. I think it is more important that each injector has a good spray pattern, with no dribbling or streamers. As far as cyl-cyl balance, I expect the volume delivered by each injection pump piston is the most critical element.