The OM603 series diesels (W124, '86, '87) use a fuel heater, it uses hot coolant to heat the fuel. I believe that this could be related to increasing engine efficiency.
There is also a difference in the bore and stroke made when they added the 6th cylinder but kept the displacement at 3l, the engine has less of a square ratio of diameter to throw.
My OM603 engine also warms up a LOT faster than any OM617 I have ever driven, I haven't figured out how this was accomplished, must be thermostat related.
The IP might also be designed to be more efficient and vehicle weight and aerodynamics may also play a part in this but the fact remains fuel economy is better in my '87 than any of the W123 turbo dielsels I own.

I think that higher fuel temp might just be part of the improvement, if so that makes me wonder if modifying that aspect on the earlier 5-cyliner engines would be effective?
DDH

__________________
'95 E320 Wagon my favorite road car. '99 E300D wolf in sheeps body, '87 300D Sportline suspension, '79 300TD w/ 617.952 engine at 367,750 and counting!