Hi Den,
you should find that your mpg should improve as the remap will improve the efficiency of the engine. Your best bet is speaking to a company who specialises in remaps.
As the Tuner should tailor the map to the car. Each car is different due to age, mileage and driving style and as such the map needs tailoring as such.
Also make sure that your car is well maintained as any issues could be accelerated due to the potential extra stress pit on the components from increasing the power.
Cheers
steve