Not familiar with that particular engine, but most diesels have a fuel screw somewhere on the injector system that can be turned up. It will add a few ponies without having much effect on MPG. The next item is the turbo. An upgrade there can again gain HP with very little MPG effect. Larger injectors will add still more but will reduce MPG somewhat. As long as none of these moves are too radical, they can collectively yield a pretty impressive increase in performance.
But...keep in mind the Allison 500 series are only good to about 250 HP. Any more and the engine will eat it. Likewise the torque and vehicle weight. See the specs below.
That's on the engine side.
Then there is gearing and tire size. Larger diameter tires will increase MPH at any given RPM.
Gearing, both in the tranny final output and the rear end can give you several more MPH. But here too there are limits. Gearing that is too tall or tires that are too big will require a significant increase in torque to gain any benefit. There are also aftermarket overdrive units that can put a few more MPH on the speedo.
A good place to compare these numbers is on the calculator at the bottom of the page.
Nearly all diesel engines can be easily tweaked to gain HP, just don't go overboard without updating all the other systems...including brakes. Remember they were matched to the other specs as well.
Allison 545 Specs:
ALLISON AT 545 TRANSMISSION SPECIFICATION - Topic
Engine RPM Calculator