The "50%" increase is for aftermarket turbo systems, which use more aggressive fuel settings and a bit more boost (6-7 on the factory IDI turbo stock vs 10+ on the aftermarket). Ford deliberately detuned the IDI turbo for various reasons: a warranty safety factor being the main one. Also, the Power Stroke was in development... in fact nearing the end of the development process... at that time. With minor tuning, the IDI turbo could equal the initial output of the new, whizbang Power Stroke (and have sufficient durability IMO). Oh, no... can't have that! Can't upstage the new PSD! So the IDI was deliberately turned back a bit from it's true potential.
Of course, the PSD as a design has much more power potential than the IDI but the first generation PSD engines were conservatively tuned because they were new. They gradually moved up the output foodchain, eventually beyond the reliable output (meaning the power level at which a manufacturer is willing to stick it's neck out) of an IDI. In general, the power potential of an IDI combustion chamber is lower than a DI (DI breathes better and has a more controllable combustion process) but emissions is the main reason why IDI diesels of all types and brands left the scene. Similar story for two-stroke diesels. It would be interesting to see how electronics could improve a 6.9 or 7.3L IDI. Imagine a common rail, electronic IDI! Infinitely tunable and adaptable!