Just for fun I'll share my experience with testing a K&N setup on my 300SD for 2 weeks in phoenix a few summers ago. First of all, yes there was quite a noticeable increase in power. More noticeable in fact was the drop in power (and noise) when I went back to the stock setup. This was undeniable, no dyno testing necessary. I also built a custom air intake to shroud and blow fresh air directly on the cone, and a remote oil separator for the breather. Second, it was much louder than the stock setup at all speeds. This actually became aggravating for me and was the reason I went back to the stock setup. Third, when I did return to the stock setup I found oily sand on the interior lip of the turbo, which I'm reminded of every time I think of K&N.
If I were souping up my F-Dude jacked-up, chipped out, black smoke at the stoplight, straight 6" exhaust, construction-job-sunburn-around-my-knock-off-oakley-ass-glasses, soon-to-be-reposessed with 8k worth of rims, chick-sitting-in-the-middle truck, then sure, nothing lost. Most people don't keep cars long these days anyway.

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'79 300D, Ivory "Gunther" ......going
'81 300SD, Metallic Blue Silver, The Grey Ghost.....going
'87 300D Turbo ...gone
'05 CDI.... gone