I ordered a K&N High-flow (panel) Air Filter from Phil and went to pick it up a week ago. At $98, it was much cheaper than what Autobarn quoted me (@ $140). Installed it on the spot at his place as well (10pm, had to test out who had the better light output from our mobiles). First thing I noticed once I started the engine and gave it a rev, it sounded the same as before (i.e. with the mutilated airbox). However, upon driving it, I felt it being much smoother. Maybe it was coincidence and I was rev-matching spot on while I was up- and down-shifting, but I definitely did not feel a lot of chugging. After a week, it still feels smooth. I don't know what brings this or if it's just a feeling of mine, but it definitely feels slightly more comfortable while cruising around.
I've opened up the throttle a few times and it sounds more aggressive than before, but not too much to arouse attention. Stock exhaust means none of that anyway :P
Phil also ordered a K&N filter for his new Ford Falcon XR8 GT. He gave it a spin around the block and I can tell you that it gooooes. Damn scary with him behind the wheel, taking side streets and weaving in and out of roundabouts at 80km/h. It's only run a few hundred kilometres, so he was actually going easy with it, never passing 4500 RPM, but being a V8, the extra revs aren't required :P That said, he still ripped a burnout and, remembering what he said about the car still being in its run-in period, took a glance at the tacho only to see it hit 6000+ RPM. So much for taking it easy!