I think they might be over exaggerating things a little (men over exaggerating?). Chipped, I could get 22-23 MPG on the econ setting on the highway. You definitely lose some power though. If you are cruising across Kansas or Texas, that doesn't matter much. -- If someone refers to herself / himself as a "guru", they probably aren't.
I wouldn't put too much faith in what they are telling you. You can get some excellent mileage with a diesel, but they are telling you they are nearly tripling it with some modifications? Physics makes that unlikely.