With my '90 300E, I have obtained a consistant 25-27 mpg at 75-90 mph with the A/C on. I use synthetic oil (Mobil 1 10w30) and a K&N air filter.
I don't think I would fool with the ignition system, but those MSD systems or Jacobs Omni Magnum units look interesting....I think some have reported that they helped (comments, Donald?).
You could probably help by lowering the car a bit, but I'm not sure what would be involved and if it would be worth it.
I think the synthetic oil might be your best bet (cost vs benefit). It's not a big investment to try it out, and it definately should not make your mileage worse, unlike what might happen with other attempted "fixes".
Just my 2 cents. YMMV :-)