blackmercedes is right:
Check the "Performance Paddock" and "Hot Rod" forums...there are tons of threads from W124 owners wanting to get more HP from their rides.
The general consensus however, is that MB built a pretty balanced product in the first place, and not much can be done in the "bolt-on" department that will yield anything more than miniscule improvements.
There are ways to get a bit more out of the M103/104 series motors, but the price for the performance is exorbitantly high!!!
You could buy a 400 (or even a 500E) for less than what it would cost to do the mods...
2009 ML350 (84K) - Family vehicle
2001 CLK430 Cabriolet (71K) - Wife's car
2005 BMW 645CI (124K) - My daily driver
2012 Mustang V6 (60K) - Daughter's car