I just read a great article on this by a professional tuner in AIM. In essence, his point was a leaner/hotter running stock Harley will auto adjust a smidge to work with either a slip-on and/or an air cleaner, but it will still be a leaner/hotter running Harley.

The auto adjust range is so limited that it will not allow you to maximize the gains of a slip-on and/or air cleaner without a PCV or the like and achieve a richer/cooler running Harley.

 

True, but there is a huge volume difference between a K&N air filter and a K&N air cleaner. Different animals, and the latter wants a air/fuel remap that the limited range of Harley's auto adjust ECM can't give.