Biggest #1 cause of lack of "peppiness" on a Mercedes turbodiesel is lack of fuel off-idle. The typical cure is to adjust the ALDA (turn setscrew CCW 0.5-2.0 turns, stop when you don't feel any more power gains on yourbutt dyno). This is particularly true for off-idle power. For a big loss in top-end power, the usual cause is a blocked banjo fitting for the ALDA line at the rear of the intake, and a blocked switchover valve.
After those 2 things have been fixed, and it's STILL slow, then mess with the timing chain & cam timing. IMO, pump timing affects MPG more than anything else, doesn't seem to do much for power. Advancing my pump timing to the far side of spec increased MPG measurably. Of course we're assuming clean filters, brakes not dragging, etc.
Check your car on a flat road from 0-60 with a stopwatch. It helps to confirm speedo accuracy with a GPS as well, since most speedos are a good 1-3mph off, which can translate into a 10% skew on your results. Factory spec for a 123.133 (82-85 sedan) is about 13.5 seconds with two occupants. Note that above 2-3000ft elevation, times will slow down quite a bit. Data from my car is here:
http://www.meimann.com/docs/mercedes/84_300D_performance.txt
HTH,