considering most "cold air intakes" in PK such in hot air from the engine bay i doubt they will be of any use. specially since they don't filter anything less than 50microns,.. which is worse than a stock air filter.
best way to improve fuel economy is to be gently with your right foot, remove all excess weight from the car, fit light alloy wheels and skinny tyres and do not fit anything to the car which will disrupt the aerodynamics.