I can tell you with absolute certainty that the 350 with dual air boxes and the 370 take the exact same filter size; if you have two K&N filters in your 350 you can drop them right into your 370 (I'm not sure about the earlier 350s with one air box; I've never had reason to check into it.
As to whether they reduce performance I think that's a bit of an untested/undocumented theory but for me at least, it sort of misses the point...I use K&N filters because I believe they provide better filtration without overly reducing airflow.
Obviously, however, a dirty filter, K&N or otherwise, is a BAD THING so whichever you use, keep them clean or keep them replaced on a regular basis!
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Robert
-My Basset Hound Is Smarter Than Your Honor Student and Barack Obama -
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