MB Doc said something about the heads a while back to me when I was getting a spare for my 300SEL. I believe it was not to mix late and early heads for some reason. May want to also drop him a line and ask.
Cams changed over the years, yes. I heard the early M103s had a hotter cam than the later ones.
Also around '89/90 the heads supposedly received better valve guides along with the EGR and Air injection system you know of too.
My question is this: why are you wanting to go through all this hassle?
If it runs fine, drive it.