Your gas mileage at highway cruise should remain about the same no matter what mods you've done to a mild car, with stock heads and bottom end. If you have 50# injectors and big turbo, the fuel to air ration should stay about the same at highway cruise as it did stock, otherwise you'd be driving around rich all the time and out of tune.
When you floor the pedal, the computer is using a completely different part of the fuel and spark table than at highway cruise.
With bigger injectors, the pulsewidths at cruise will just be smaller, allowing the same amount of fuel in as the stock injectors did.....so with that said, be sure you do the computer chip mods with all your other mods to adjust for better breathing, better exhaust flow, more air, more fuel, etc..