If it is true that installing a freer flowing air filter increases power, than it is equally true that the stock air filter system is "choking" the car of power.
If that is the case, I am surprised that MB engineers were not smart enough to simply install a larger surface area filter box. It would surprise me that they would leave "free" (i.e. it doesn't cost in emissions, $$, safety, etc.) hp on the table. And, likely, also free MPG. Given the unbelievable R&D that all Mfrs put into making their engines as efficient as possible, I'd be surprised that not one of the engineers at MB has ever thought to test their stock airbox/filter system to see if it is choking the engine, given that if that is the case, a solution is so simple.
It has been proven by the most reputable Porsche mechanics in the world (Bruce Anderson, Jerry Woods, and others), using dynos and multiple tests, that with VERY few exceptions, Porsche engineers have been smart enough for decades to install large enough air filter boxes on their cars such that the stock air filter is not a bottleneck. They have undisputedly proven that a K&N gives no increase on a Porsche. In fact, they have even dyno tested with NO air filter at all, and shown no increase on most Porsches. The simple fact is that Porsche was smart enough to equip their cars with air filters/boxes that are ample to supply the stock engine with far more air than it can ever use (if you are talking about modified engines, of course none of this counts).
Additionally, K&N's stats (I've e-mailed them directly to get the info) never claim to filter as effectively as the stock filter. The best they have claimed, that I have seen, is that a stock paper filter will filter 98% of particulates, while a K&N does 97% in the same test (tests run by or on behalf of K&N). That is a deceiving number. It does not mean that a K&N only filters 1% less, in fact, it means it is about 33% less effective. Of all the millions of particles in the air, the stock only lets in 2 out of every 100. The K&N lets in 3 out of every 100, an increase of at least 33% (or maybe 50%, I'm not very good at math).
So, in conclusion, I think it is remotely possible that such a thing may increase power marginally on a poorly engineered car. I'd be surprised if it does on a well engineered car. In the absense of PROOF (valid, third party testing from a reliable, disinterested third party), I am willing to give MB the presumption that they were smart enough to not choke their engines with a too-restrictive air filter. That presumption can be overcome, but only by real, reliable evidence. Thus far, I've never seen it.
Weighing against that is the fact the K&N has NEVER, to my knowledge, claimed that their filters actually filter dirt as well as a stock filter (I's suspect they won't make that claim, for liability and false advertising reasons, because it appears that their testing indicates that their filters DON'T filter as well as factory paper filters).
(They can easily get around the false advertising rap on their HP claims, because you have to carefully read their ads to see what they actually claim. They claim "up to X hp increase", but don't ever specify on what car, or what conditions. They also generally only claim their filters flow more air, but that is meaningless. If the stock filter flows X, but the engine can't use more than X (as is the case in most well engineered cars), than increasing the flow of the filter to X + 1,000,000 will do nothing).
Just my opinion.