If you test your 603, I'm curious to hear the results as well. From the anecdotes I've read, in general most 60x engines produce less soot than most 61x engines. (There are always exceptions, but I mean in general, assuming similar conditions etc.) I'm not really sure WHY that is though, since the basic design is the same.

Side note: Engines with direct injection, like the Cummins, produce almost zero soot - but they are obnoxiously noisy. MB engineers chose indirect injection mostly because it's quiet, AFAIK, and decided that higher soot levels was an acceptable compromise for the decibel reduction.