A properly matched turbo system can and WILL increase your fuel mileage. How do I know???? I actually speak from experience.
My 85 GT was converted to MAF EFI, and with the stock 302, full 2.5" off road exhaust, stock 3.08 gears, 5 speed, 255lph high flow intank pump and 1.7 roller rockers used to get between 340-350km per tank. That's about 220 miles to a tank, and I never drove miss daisy. This is with a fully functional EFI system and everything hooked up, no smog.
I converted to EFI turbo, with a CRAP C+L maf and 30# injectors. The car NEVER ran right with this maf, and I never got around to changing it. I also never wound up installing the O2 sensors in the system. In this set up, the car used to get between 340-350km per tank. I got the exact same mileage with a 'malfunctioning' and un tuned efi system and through Flowbuster mufflers! My turbo set up had so much more potential, I just used to have a money-vampire GF at the time so she got more spent on her than my true baby (don't give me ####)
On the other hand, my friends 95 GT with GT40 aluminum heads, stock cam, 1.7 rockers, Innovative T61E turbo with a nice custom turbo system, 50lb injectors and 80mm maf, no tune, full exhaust, fully ported Cobra intake and 70mm TB...... he used to get 500km per tank easily (312 miles) and once managed 700km to a tank all highway. That's 437 miles to a tank! I see lots of guys with mild N/A set ups around here talking about 180-220 miles per tank! If you don't get into the boost, it will be more efficient than naturally aspirated. And efficiency is what saves you money.
'86 GT T-Top - 351W/T5/Ethanol - in pieces.
'88 GT Vert - low mile 5.0, bowl blended E7s, GT40 intake, full exhaust, 3.73s, T5, 17" '03 Cobra replicas.
'89 LX Vert - non-HO 5.0/AOD, getting boost and a proper motor, spray bombed satin black.