I just installed the K&N 57-1553 and have been happy with the immediate deep throaty sound when I throttle mash.
Kudo's to the installation instructions and many K&N Youtube vids.. Well put together and made the install easy. Was a 2 beer job.
I went without the throttle body spacer .. didn't see the need.
Next investment is the Metal Mulisha Gibson exhaust .. love that this is available in baked black