ya need to have your fuel pressures checked cold and hot.
before this, get another mechanic that knows this system.
needing hotter plugs to burn the extra fuel tells me this guy
has no clue. stock plugs are more than adequit. do not use
platinum in this engine.
a c.o. tester can tell you if you have to much gas or are having
a lean miss problem.
are the plug connectors ok?
on and on. get a new mechanic first that knows mercedes.