K&N's are another thing where you get mixed reviews. Some say they don't filter ultra fine particulate as well as they should. Even if they are totally benign, the most gain you will get will be 5HP or so. I'm not sure that small an amount is worth swapping out of stock. Performance and longevity is probably better served by keeping a OEM filter element in but changing it twice as often. You will get a bit more intake noise so if it is a sportier "sound" your after then the K&N will suffice. I would certainly avoid those "cones" that go underhood. They take in too much hot air from underhood and actually reduce power by about 3-5%. The stock airbox works well enough. (I guarantee that the MB engineers had a bigger budget to design the intake than K&N did) The only redesign I would consider would be engineering the newer V8 "twin-tube" design off the CLK430, S500, etc onto the V6. Not the single plenum from the E430 but the twin tube as seen on the W124 V8's, and the models mentioned above. *See MBshop reviews* The bigger volume OEM airbox for the V8's (in twin-tube dress) would likely maximize breathing potential without being a hazard.
Just a thought...Lee