remap is about £400 to £500, revo and custom code are popular choices and have there own sections on this site. revo is more popular with the petrol peeps, but CC has a smoother torque curve to avoid clutch issues, may seem pricey for a bit of software but you get that back in cheaper insurence and paying less for the car in the first place. as for MPG if driven very very hard you with see a small decrease in MPG, but if driven normally
you can see figures increase due to reaching cruising speed quicker as your not accelerating as much