It depends on the remap in question.
Here in the states, we have a car here called the Chevrolet Cruze Eco (I know you get the Cruze over there, but if I recall, it's diesel only. Please correct me if I'm wrong). The car is equipped with a turbocharged 1.4L gasoline direct injection 4 cylinder, and is rated at 42 MPG (miles per gallon) highway when equipped with the 6 speed manual. A company called "Trifecta Performance" offers a mail order remap for the stock ECM that ups the power levels from 138 HP and 148 ft-lbs to 170 HP and 200 ft-lbs (dyno proven). This remap also increase the fuel efficiency as noted by everyone who's installed it, with 55 MPG being pretty common. That's just an example but like I said, it depends on the remap.