I may not have data, but as a owner of several different types of vehicles. I'd like to attest to K&N, ITG Foam, Macht Schnell and CGS now.
Let me start with the K&N, when I purchased my 2003 325i from a friend he already had the K&N installed. I filter maybe had about 25k miles on it when I swapped it out with a Jim Conforti Air Intake with an ITG Foam filter.
I sold my Stock OEM airbox and filter with it, when I went to wash it i could clearly see all the debris it had captured. It was very easy to clean and the material it is constructed with is top notch. I mean the metal weaved into holding the filter in place rubber surround mold where excellent after my use. I was impressed and for the value it does pay off. I mean they average about 40-50 dollars, it pays off quickly after you replace the filter elements by the third or fourth time.
I hear the improvement in performance, I may not feel it but in the end it has proven to be very reliable.
ITG Foam; well where do I start. I purchased the intake from a fellow enthusiast, who knows how many miles it had on it. I serviced it one time, when I decided to service it by washing, drying and attempted to recharge. BAM it started to fall apart, I mean chunks started to just fall out kinda like a balding white male in his 40s....ahem.
So there I was freaking out because I couldn't find a replacement quick enough. I had to order it and wait a day or two. ITG Foam filters are not the easiest to source in the US only a handful of Tuners in the country carry them. Atleast if you own european vehicals they are a ***** to find.
The performance I found to be great, the filtration is evident. As I am a DIYer and do my own services I can happily report on over 50k miles of use that I (knock on wood) have not encountered any dirty MAF sensors or need to replace them due to saturation of re-charge oil for the filters. I'm about to hit 100k miles on my 325i I purchased it with only 17k on the clock maybe less.
ITG Foams are not too expensive but do the job and overall I'd say that the cost of maintaning it yourself pretty much equals that of a filter element. I personally with my experience in handling them I'd do it over again, I don't like paper. Due to the air paritculs that get trapped but when it continues to accumulate eventually after the filter has filled it will pass dirt with the air flow. As for it being oiled it traps it and chunks it and keeps it from passing the filter element preventing it from reaching the throttle bodies or MAF sensor.
I loved the Macht Schnell (Make Fast translation from German to English), it was the first air filter replacement for my 2009 M3, the quality surpassed that of the K&N. The sound it put out and the filtration were incredible. The money I spent on it about $99 dollars easily payed for itself when it came to servicing and replacing paper filter, they were quite expensive for the V8 engine on that BMW.
The CGS air filter is of K&N quality, the construction is very sturdy and of great materials. I wouldn't doubt if its a re-badged K&N filter. The way our MAF sensor is protected I doubt that it would ever get damaged unless you didn't have a filter on it. The reason I say this is because the MAF sensor is enclosed in a retangular box that is right in the middle of the intake. I like this intake because it will be very easy to take off and clean. All you need is a screw driver off/on wash, dry, and recharge.
Plus if the sound alone does not in turn prove that the OEM is restrictive and inefficient that I don't know what is. The Ex pulls quicker and with a 50% increase in throttle response it GOs!! I'll let you know how that works out in about 10k when I see if it needs to be washed and serviced.
I hope that helps you. I have always used Super Duper Unleaded in my BMWs and still to this day they are running great. My MPGs alone on the 325i are up to 33 consistent on the highway. I believe it was originally rated for only 28-29 on the highway.
I Sir, prefer my oiled filters overall paper dry.
