[skyking]

Don't set your goals lofty high with that one. As everybody says, the utility costs. Since you have a great MPG rig for daily driving, I would not go crazy with efforts on the subie.
I'd recommend cleaning off that rack drag, and making a nifty removeable boat tail for just the top part. That will yield the most improvement for effort. Bonus is it won't detract from the offroad capabilities.
coroplast underneath is going to be problematic if you play in the dirt. It will build up on it and you end up adding a ton of weight or losing it outright with load. It is amazing where the dirt has build up in my ram truck, we ar talking tens of pounds of concrete like hardened mud that I have to chip out.