They're good for way more than 6k miles of normal driving climates! If it was dirty enough to rob any power whatsoever, you would have a check engine
Placebo effect.... Confirmed.
Think how much placebo power you would feel if you put in a new filter, added a bottle of octane boost, put on fresh wax and ran nitrogen in your tires!
There's actually something to this. Havnt you ever thought your car drove better after you detailed it?