I swapped the head on our 300E some time ago. The original had no problem at like 130K when we did this, but I wanted to try the polishing trick. I had a spare head and polished it my self with a Dremel tool, didn't port it though. It did made a nice improvement, although I think it was mostly from removing all the carbon deposits in there. The car had little improvement at low RPMS, but as the RPMS went up you could feel the change; specially on second gear, and those 4 to 2 kickdowns.
If you can get it done for a reasonable price, I'll say go for it; or at least make sure they clean al the carbon deposits. It defenetly makes a difference on a highmileage car...

my .02

__________________
'02 C240
'00 LR Discovery
'72 280SE
'67 280SL

---past---

'79 280E sold (RIP)
'86 300E sold
'87 300SDL sold
'90 Laforza sold
'95 320SE sold
'98 ML 320 (sold)