Well, you are being a bit unfair. Comparing the body of the W123 to the body of the W124 is comparing apples to oranges.

Let's compare the W126 body. I've got one 617 and one 603. That's fair.

The SDL consistently gets 28-29 mpg.

The SD consistently gets 25-26 mpg.

Hopefully the new valves, 2.88 diff, adjustment of injection timing, recalibrated injectors, and proper valve lash will get the SD up closer to the SDL, but, I don't think it will ever catch it.