That's a good looking truck you have, wanna trade?!
I'd say leave it. For two reasons:
1) Because of the old saying 'if it ain't broke...'
2) If it's your heavy duty work truck (towing, etc.), then it is what it is and if speed is your biggest demon, I would imagine while towing, you would be driving slower anyway so you would be fine. You have the Lupo for all other situations.
You could work on some aerodynamic improvements to increase FE. You could do a trailer like BZP:
Single Wheel Trailer Boattail.
I'm planning on a roof rack for my 4Runner. Obviously it will be good for cargo, but the plan is to make it to match the aero-template. I think I'm going to use some EMT conduit and bend it to whatever curve/angle the template calls for and then I want to attach a kammback to the end as well. It will increase frontal area, but decrease drag. I might also put the spare tire on the rack, meaning I can clean up the underside of the truck as well with a belly pan. By doing it this way, everything is reversible, if it ever needed to be changed.