At long last I'm giving up
After following all the troubleshooting advice you awesome fellows have offered, I still haven't been able to find the culprit behind my lack of power issue.
The turbo outputs 15+ PSI under load (tested via intake manifold boost pressure line output), boost pressure line and vacuum lines are clear (and match the 603.96x vacuum diagram supplied by sixto), the car starts instantly and idles perfectly with literally zero engine shake, both fuel filters replaced, new air filter, accelerator pedal engages the throttle to its fullest throw, and finally, the non-engine peripherals (transmission, fuel tank/tank screen, waste gate actuator on firewall, and everything else not bolted to the engine) are ORIGINAL since before installing the 'new' engine (#22 head) that exhibits this problem. In other words the car was a ROCKET with all the same components and a different engine+IP (original, #14 head).
To elaborate on the performance problem, the car lacks power
evenly and consistently at ALL rpms. She's extremely sluggish from a stop and accelerates sluggishly at all speeds. It can reach 80+mph on a flat freeway, but it's slower than my 1980 non-turbo 300TD. I can hear the turbo spooling up at the normal ~2500 rpms and can BARELY feel its affect. Up-shifting is late in all gears. There is no engine faltering/hesitation/missing or that feeling of restricted fuel flow at high RPMs like a clogged filter would cause.
Onager Corporation at 789 Energy Way in Chula Vista, CA just called me back about the car saying that THEY give up in trying to find a solution, without even charging me one cent. They only work on 1950s to 1980s Mercedes, have a very good reputation here on the forums, and were completely honest with me - at first they were going to do a compression and injector test but when the owner Bob went through the car he said there's no way those tests would lead to a solution, that it would be a waste of money for me. He was stumped. Leads me to think the IP is at fault, which I know is rare, but there's nothing else left right?
Here's the craigslist ad I just posted for anyone interested in parts or who wants to continue the project:
http://sandiego.craigslist.org/csd/cto/4121497673.html
I'm willing to start parting her out if that's what it takes. I guess this is my last ditch effort to see if I missed anything. Thanks so much for all the help you guys have given me! I'll probably be reverting to a W123 wagon, as my 1980 was so awesomely reliable.
Cheers,
Daniel