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.
It depends how you drive it, if you take advantage of the extra power on offer then it will be lower. If you drive at the same speeds/throttle style as before it will usually be improved.
Diesels nearly always improve noticeably on MPG after a remap! Petrols generally show a marginal improvement but it does depend on the parameters of the remap. Some people specify better mpg at lower RPM and more power at the top end which seems sensible to me!