Diesel fuel economy is inversely proportional to the amount of power being used - not the amount of power its capable of. The more power you use - the lower the fuel economy numbers. The baseline fuel economy of say just driving around on backroads and using similar power to a stock truck will be determined by the specific setup itself. If you have the truck setup to be efficient in that range then it could do decent. Say your path to 1000rwhp involves just a single HX-82 turbo - its going to suck to drive and get ****ty fuel economy. Now if your path involves a reasonable set of compounds or triples that would likely get better fuel economy. Of course I'm painting rather broad strokes with this statement, so your mileage my vary. LOL
Good hear you again Jason.