for me, the grill block hasnt increased operating temps despite completely blocking off an opening close to 2 sq ft. it gets all the air it needs and more from the air dam... so mines only getting me the aero effects right now. but i've been thinking about adding to the undertray from the air dam forward, and part of it would be to limit the opening up to the radiator.
i never did a decent a-b-a because of the massive amount of tape i used for my first grill block, my current stealthy grill block requires more work to remove and reinstall than i'd car to do for fun... but from my older fuel logs when i put it on i'd say somewhere between 0% to 5% improvement
metrompg tested one at 2.9% improvement
Testing grille blocking & wheel skirts: +5.7% improvement - MetroMPG.com