I don't know the OM606, but sounds strange if new nozzles in existing housings lowers the pop pressure that much. In my OM617.952, pop pressure should be 1950 psig, which was close to what I measured in all except 3 injectors "as found" in my 1984 300D, which popped at 1600 psig. That was correct for the non-turbo engine and I recall was also marked on the housings (in bars) which were black instead of olive colored. I then installed the full set of 5 injectors from my failed 1985 engine in my 1984 (after tweaking pop pressures to 1950 psig). Strangely, it made no noticeable difference in how the engine ran. Makes me wonder why people fuss so much over exact pop pressures. I suspect that spray pattern is more important.

I don't know if you can get there by shimming. I bought a bunch of generic shim washers from McMaster-Carr, but recall using only the 10 mil thick ones in tweaking. Adding one would increase pop pressure ~100 psig as I recall, which was the most I ever needed. Some people also sand the factory disc, but I wouldn't molest it and no need to be than anal. Going up, you need to worry if you will go closed-coil on the spring to limit how far the nozzle opens. Perhaps that could damage your IP if too much back-pressure then builds up at max flowrate. If like my injectors, and my memory is correct, you would need to add 45 mil thickness, which seems like a lot of extra spring compression (about a spark plug gap). Perhaps, instead you need different springs to match the nozzles. I wonder if anyone has ever tried stretching a spring to make it a little longer and increase its pre-load force when installed. Could be dangerous (fly loose, into your eye or neck).

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1984 & 1985 CA 300D's
1964 & 65 Mopar's - Valiant, Dart, Newport
1996 & 2002 Chrysler minivans