Tuning choices for a GMC W4 Box truck

I know this seems like an odd question, but with so many of these trucks out there, it may be more available than I know of. My father-in-law has a 2005 GMC W4 box truck with a 4 cylinder turbo diesel engine in it. He installs Allen digital church organs and the associated speakers, which are obviosly pretty heavy. Thay average about 8mpg in it, and would like to know what choices they might have to get a little more power out of it, and possibly raise the economy a little.

Short of putting cones on the front and rear of the truck, any ideas?