I am wondering what has been the best high performance air filter in your experience for a 2012 GLK350? I was reading mixed reviews of K&N Air Filter. Are there any cons for using an K&N?

 

The Mann OE filter is the best fliter - there are zero reports (so far) in 9 years of a K&N filter giving any performance improvement for a Mercedes - frankly other than "psychological"

 

To put it simply, K&N is snake oil. The stock filters are not restrictive and do an excellent job of keeping damaging particles out of the engine. On the other hand the K&N is more porous and will allow more fine particles to enter the engine. The K&N must also be oiled periodically and the oil has a bad reputation for fouling MAF sensors in the intake. The stock filters require no maintenance for their lifetime.

Your engine is computer controlled to maximize efficiency of the combustion while limiting emissions. Even if the K&N did have a minor flow advantage over the stock filter (many studies show the opposite is true) it would have no effect because the engine management computer is constantly optimizing the mixture anyway.

If you are looking for performance gains, look at a tune up and general maintenance. If that isn't enough then maybe look at the AMG models with the bigger engines.

 

^^^ This. It’s also one of the best methods to kill a MAF sensor, which aren’t cheap.

OP: stick to OEM filters (and other parts) and your Merc will thank you.