Are you sure the Woodruff keys have no offset?
Mikaelw - when you had the cam gears removed, did you check that the Woodruff keys which locate in the slot on the cam shaft and on the gear wheel are straight?
MB sells them in various offsets up to 10 degrees and it's possible a previous owner/mechanic compensated for bad timing by installing offset keys. Offset can be positive or negative, depending how you install these.
With a new chain and tensioner, you should be using straight keys on both cams, not dog-legged ones. Offset keys would continue to return a timing error.