I ask about the same question about a year ago, No one replied.
On farm tractors we have advanced the pump a degree or 2, adjusted the max RPM's 1 or 200 rpm's, on some of the older Bosch pumps it is real EZ to turn the fuel up.
A good Injector shop might shed some light on the subject for you. My way of thinking 2 or 3 horsepower would be great, about a 10% increase.
We turned up an old IH D268 engine, dynoed @ 91 HP pto @ 3200 engine RPM, in all Honesty, I don't think the dyno operator calibrated the Old M & W dyno correctly.. But it sure pulled good and blew heavy Black Smoke, HaHa