I love how quiet my klx is, just makes everybody so much nicer when you ride up on them on a trail( I'm not supposed to be riding on). I drilled a bunch of 1/4" holes in my lid,it's much quieter than with lid off and I'm sure it's breathing well.
marc

 

Air box changes are more likely to need jetting changes to compensate than any other mods, if performance is lost it's due to an undesirable change to the afr.