To answer your basic question, it is unlikely that you will find anyone to do dyno comparisons to MBs performance specifications.
The best way to check it's performance is to take it to an experienced tech and have him drive it. Having driven thousands of 500SELs I could tell you in a five minutre test drive whether you were in specs.
As to fuel pressure the first thing you need to do is figure out what fuel pressure means to your car. if you think you will get any sort of indication of fuel pressure by looking at the plugs, you are a long way from needing gauges.
Bosch Master, ASE Master, L1
33 years MB technician