Yep, the OM603 engines are remarkably efficient at higher speeds, plus the W124 chassis has a lower cd as well. From my experience, the MPG doesn't change drastically up to at least 90mph, perhaps 100mph. I don't know of anywhere in the USA that I can do extended periods of 90-100mph to find out. Oddly, the MPG doesn't improve much at lower speeds, it seems pretty constant between about 60-85mph.
BTW, the W124 is geared lower, at 75mph it's 3000rpm, at 100mph it's 4000rpm. I did a top speed run last weekend and saw 120mph and it was still pulling, I backed off due to a car coming - didn't want to chance it being the fuzz.
The best I've ever heard of from a W123 turbo is 30-32 but that was RARE and I have no idea at what speeds those numbers were attained.