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The OM603 engine is actually more efficient than the OM617, so no, you won't see an increase in MPG. They might be about equal, although it seems the OM617 tends to get slightly lower MPG unless you drive very conservatively.
The OM603 appears to have almost no MPG penalty at higher speeds, the best MPG I ever got on mine was 30-31, and that was at a steady 80-95 MPH in Idaho & Montana. My normal highway mileage (70-80mph) was more like 28. Weird. Also, the W126/W124 cars with OM603's usually have taller rear ends, due to the extra power availble.
For all OM617 & OM603's, don't expect more than upper 20's or low 30's at BEST. If you want more, you need to get a 190D with a 5-speed, these routinely get 30+, can be coaxed up to nearly 40mpg!! The 84-85 4-cylinder models get better MPG but the power is kind of weak compared to the 86-up 5-cylinder.
HTH,
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