Like many others here I suspect, I am a diesel "Newbie" but I'm going to give my opinion anyhow!
It is my observation that these vintage '75-85 MBZ diesels can be out of time due to chain wear or otherwise a considerable amount before you will finally know it. When you have an engine that runs best at 24 Deg. BTDC, well that's a great deal more advance than any gas engine I ever heard of so that's a definite difference and as I see it this large amount of advance " in-and-of-itself " means that there is is greater latitude for things to get " out of wack/spec " before you are going to notice.
If we were to start comparing the %age of timing deviation from "spec" for both gas and diesel [deviation before you begin to notice what we might consider to be a problem] well I think we would see greater similarities between the two. Sorry about this abstract thinking.
To reinforce what I have tried to say above let me relate that while working with my new found "Indy MBZ mechanic" friend from Hungary, using easy-to-setup-and-use" electronic piezo-pulse sensor timing equipment I have observed quite a few MBZ diesels from 1975 to 1988 that were considerably out of time [up to 10 Degrees] and no one seemed the wiser! So this tells me that at least these older diesels have been running this way for a long time and will no doubt keep chugging along that way without being diagnosed as needing attention!
My goal remains to help my friend put together the means to quickly detect such "out-of-spec" timing situations and to adjust the IP in 1 to 1.5 hours total labor. So finally to your question...
I would suspect timing first on a car with a top-end speed problem that just does not seem to be solveable by any of the other normal things often done by mechanics attempting to solve this somewhat common problem for these cars. Don't ask me for any deep engineering explanationfor this for this is strictly based upon my own experience with my 240D which has a very young engine.
I estimate that the straight-and-level top-end for this MBZ improved ~8 to 10 mph when I changed the timing from 20 to 23.5 Deg. BTDC.
I rest my case... no let me change that to " my speculation ! ",
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