I've heard the same thing. I have a K&N in my truck, because it was in here when I bought it. I also just dismantled my turbo to upgrade it and saw nothing bad at all. There was no residues or anything. I've just been lazy and haven't gotten the BHAF yet.
BHAF= Big Honkin Air Filter. They are just an extra big filter that fits our air intake. They require a special home brewed mount of one sort or another. The "old smokey mount" is popular. I'll try and hook you up with more info in a moment.