OlDPhoneman,
I drove a 99 Volvo w/M11 400 bobtail from SLC to the Oregon State line, then back across Idaho, Utah, Wyoming, Nebraska and Missouri to KC. The speed limit through most of those States is 75. I cruised 77 where traffic allowed. I got 11+ MPG overall. Not too bad.
If you want some serious HP, check out the site below. This guy also also wrote a book called "Horse power and Torque" It's an interesting read. He primarily builds high HP mechanical Cummins engines as opposed to Electronic Cummins engines. Still, he will know what you can do and what it will cost.
www.dieselinjection.net
later,
Larry