found this on NABISCO. my '11 outback also has one as PZEV is mandatory in washington. i found something but am very skeptical
"Don't fall for the "PZEV"(what's the sense for a 2.0l car when the monsters with the 5.7l don't have it?!) - get rid of the additional charcoal air filter
(just break and remove it, no other way) and replace the original with a K&N filter. I can't describe the difference! The car accelerates much faster, you feel the power and on top of that - 3-5 mpg more, especially on the hwy! Had a 900 miles trip to Champaign-IL, averaged 38 mpg on a 65 mph average-how's that for the 36 mpg at 55 mph official data!? "
i guess the only difference is the engine will be a little louder. is there any downside to removing it? you have to break it...