I am in similar boat as you with mileage. To be honest i think its the gas! Granted i could do a bit more to improve it, right now i average about 12mpg mixed driving but 16-17 doing 75-80 highway and in town driving conservatively most i see is 9-11mpg.

I have a full stock 1995 F150 EFI E4OD with 3.08. I plan on getting clifford headers in next week or so which i hope will free up a few ponys and better highway with an intake as well with an electric fan. If you stay off the gas and don't use the extra ponys given try to let it breath better with a new exhaust and a intake. Also have you checked all your vacuum lines? Could be a leak and not sure about it. Check fuel pressure as well and see if it is consistent, check fuel delivery, not familiar with TFI but anything helps. Replace fuel filters, and curious as to what your driving habits are. Tire pressure also makes a huge difference. i typically keep mine at 35 but i hear if you up the pressure a bit it can help on highway but little less traction when wet and if you off road.

Just my 2 cents, take it with a grain of salt or not. Either way let us know if you find anything!