I have had good luck with my combination; Flowmaster dual exhaust, traction lock in the rear, cold air intake, throttle body spacer, 3.55 gears, and Diablosport tune. I have a '98 auto, but those mods really woke it up. When I am on a highway trip, I get nearly 30 mpg, even with the 3.55 gears. Those gears give you good passing torque at highway speeds.
I have the following mods on my 01' V-6 5 speed car: Magnaflow cat-back, Maximum Motorsports full length sub-fram connectors. H&R sport springs, Accufab 70mm throttle body, Steeda Cold Air kit, Auto-Specialties underdrive crank pulley, SCT Tuner, Ford Motorsport Track-Loc rear differential, QA-1 caster/camber plates, QA-1 Bumpsteer kit. UPR rear lower control arms, synthetic fluids throughout the drivetrain. And Boyd's 18X10 rear wheels with 275/40/18 Nitto 555 and Boyds 17X8 front rims with 225/45/17 Nitto 555.
Edit: Forgot to add the grill delete and the Mach 1 chin spoiler to the list of Modificationss
I already have a stroked 5.0 supercharged Saleen, the 01' V-6 is my DD and gets very good gas mileage. Thought it would be fun it perform a few mods on it to see what it could do without hurting the fuel mileage at all. Right now it is a total "poser".