If the valve stem seals are still inact they might mask a lot of wear in the guides. Unfortunatly a lot of mercedes engines have a lot of wear because of their accumulated milages.
I can only relate to their gas engines in this area. I have seen mercedes gas engines with no real amount of top cylinder ridge wear evident yet the bronze guides were worn to death. The inside of some guides where worn so bad the recessed projecting portion of the guide that the seal sits over was partially worn away.
Another thought was if the gentleman rotated the valve spring for example and the edge of a valve lifted off even a little the clearance should increase not decrease as he experienced.
I suspect the seats get worn in a complex way as well when the guides or guides and valve stems are poor. Almost an oblong seat enterance and exit ramp perhaps. This may have produced his results.
The valve almost has to relocate itself as it closes. It initially does not hit square on its ideal seating position. So really good sealing is also questionable. All the improvements you have seen with the head you had sent out may have been a combination of many factors. No doubt in my mind a valving system in really good condition is worth the effort..
For discussions sake. Say you have an older engine that runs pretty well with known good compression and lots of accumulated miles. You suddenly burn a valve at some point. During the repair you have all the guides, valves and seats returned to specs. The car almost always seems to run better afterwards and usually starts easier.
One of my guesses is that even if you have good compression prior to the failure with a compression gauge at very slow rotational speeds that sealing might not be duplicated at operational speeds with a lot of wear present. The deficiencey may even start to appear as soon as rpms elevate any higher than cranking speed during the start proccedure. The valve may almost have to slide into the best available closing position. There may be no consistancy of that either. An additional contributor is the old valve springs may have lost some of their strength as well. At higher rpms there may not be enough time for the valve to aquire the best available seat position. Mass/inertia problems accelerate.
One nice additional feature of the diesel is it might tollerate a minor leaking intake valve better. Since it is not back bleeding into the fuel input mixture. Of course that too may be offset by the need for greater basic compression.