Extremely common upgrade on 12v Cummins 6BT in Dodge pickups.
AFAIK, less fuel spills to the return line and more goes into the combustion chamber. Delivery valves on P7100 pumps are another common Dodge upgrade.
I can tell you that several 400hp+ 603s are running around europe with the original injectors and delivery valves. They replace the 5.5mm pump elements with 7mm, along with other pump mods, intercoolers, custom manifolds and big turbos (Holset HX40W is a favorite there).
I'm thinking of taking a similar course with my OM602. I won't do a whole lot to the pump, just put the 7mm plungers & barrels in and modify the governor to start defueling at 6000rpm. I'll probably use a Garrett GT23 VNT, which is bigger than the stock turbo and smaller than a Holset HX35, should be good for 250-275hp, with less turbo lag than a Holset or even a stock turbo. I'd rather have 250hp with predictable delivery than 330hp and major turbo lag, even if the 330hp car can beat me by a second in the 1/4 mile.
Warning:
I would not do anything to increase fueling beyond stock, or increase boost beyond 15psi, without a good exhaust gas temperature gauge. A MBZ turbo properly set to stock specs is already capable of some scary exhaust temp numbers. A turn or two on the full load stop screw could probably get you a cracked head or piston if you aren't careful. You will almost certainly need an intercooler to get a noticeable power increase without meltdown.