Not to reopen a debate that seems to be finished if not resolved, but... Members may find the following site instructive although the same information may be found at the Catapillar or John Deere sites. http://www.lugger.com/
If you look at the L1066 engine for instance, you will see that it can produce 125 HP using anywhere from 1 to 2.7 gph depending on degree of boost and intercooling or, put another way, anywhere fron 90 to 125 HP on one gph. Look at flywheel HP, propeller HP only tells how much HP a std. propeller absorbs at various RPM's. not engine output. IMHO this shows that HP gains are possible, up to a point, with additional boost and charge cooling without additional fuel.