No, its not always all in the mind. How about a 6.8 hp gain on my turbo charged 93 MR2 Just by replacing the stock box with the K&N cone. The actual K&N kit for this car is just the cone and hardware to mount it on the stock tubing. Because the car is Turbo charged and has side ram air inlets near the intake head, clearly shows why this car made 6hp so easy and NA cars cannot in most cases.
Want to say to me: "try the dyno with a paper cone filter in place of the stock box, you will still see 6.8gain" ?
I did. 2.4hp gain, If you even want to consider that a gain at all.
Almost all turbo charged applications that use a stock paper filter and box can be improved upon, sometimes by even 10-12hp in a T'charged 4cyl.
Where there is restriction, K&N always flows better the paper.
Of course its all in the mind if you throw a K&N drop in filter in vehicles that are not restricted by the box or filter.
The K&N drop in has been known to make power, (rarely) One of the few examples in the LS1, of course it was very very neglagable but what would you expect? Im pretty sure it was used in Horse Power on TV and they made somthing like 2hp in an LS1 Camaro, they had tested the filter prior to the Dry-Nitrous kit they were intalling.
If your looking for cheap power, you cant spend $50 and got a drop in. You must first check to see if K&N has your car dyno'd in there intake section, if they do, and it makes decent power, then you have to spend the $200-$300 bucks for the kit. This is where it doesnt become worth it for alot of people, but when a $200 kit makes 10hp on a honda civic Si, thats about the cheapest horsepower you will ever see next to NOS:
http://www.kandn.com/dynocharts/69-1009.jpg
You gotta pay to play gentlemen.