I would agree. Chain stretch has no doubt occurred but it would be measured in microns ;-)

From my own experience I can relate that my 88 300E Daily Driver has been a dog for most of its life. Interestingly it never suffered the dreaded EHA failure until it got to 290k miles late this past fall. I replaced plenty of parts trying to get better acceleration but never found the cause until replacing the EHA.

I had replaced two on the wife's 89 300E and never had to adjust them. But when I put the new EHA on the '88 I never thought to adjust it and didn't. The car woke up! It would chirp the tires on the third gear shift at WOT! It also, however was running too rich and overheated a cat.

I had always attributed the difference between the '89's strong acceleration to the dogginess of the '88 to the '89's low mileage (115k vs 290k at the time for the 88). But it was fuel that made the difference.

Although I believe you have gone through much of the fuel system if not all of it and have adjusted your EHA, I would suggest richening it all up to see what the reaction is.

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88 300e
89 300e
93 400e
93 500e
77 308 GTB