The 1.5 TSI uses the Milller combustion cycle which should produce reduced real world fuel consumption, by allowing earlier ignition timing and an (effectively) larger expansion stroke. This is also the reason for the small hike in capacity - the Miller cycle is slightly less volumetrically efficient. The Miller cycle also makes for reduced NOx and particulate emissions, as a secondary effect of creating a more complete fuel burn.
It also uses a variable geometry turbo to give a broader torque curve, and reduced lag.
Incremental performance improvements rather than huge leaps, but it should be better than the 1.4 in theory.