I'd say 5 at WOT @ redline. Not worth the $$ IMHO.
What we need is a CAI. I've been watching the Intake Air Temps with my ScanGauge II and see that it is 10-13 over ambient even at highway speeds. And 20+ when in town. Bottom line...our intake is a big-ass heat sink.
I had a K&N replacement filter and it doesn't make any difference. However, I replaced it with a K&N CAI from www.autoanything.com for only $280 delivered to my door. This makes a big difference, especially at around 5000rpm. I can really feel the car pull and it sounds amazing....if you like it loud.