Looks, sound, and performance!
A good set of headers will get you a 15hp gain for around $600 plus installation. $400 or less for 10hp is a deal! Exhaust cat-back might get you another 12 for $1200. Less restrictions in air flow thought system = +HP. All the way from where it sucks it up to where it blows it out.
You will read claims of 10-20 hp gain with CAI's (Cold Air Intake). I would figure 10 to be conservative. Some people have posted that they only got 10hp gain with one of the mfgs on here that is wanting to challenge all comers. I think their air filter is a little on the small side. There box also allows for more eng. compartment(hot) air to get into the filter pick up area with holes and loose fitment to fender well.
Air and water will only travel as fast as the most constricting point will allow. Sprinkler systems are designed with 2 inch trunk line feeding water to 1/2 delivery lines and all the sprinkler get the water they need. Try pushing water through a 1/2 inch line into a 2 inch. The 1/2 limits the whole system.
Look at the stock box. You will notice a smallish rectangular opening where the air enters. It then necks down to a smaller tube section leading into the box. In the box it hits more obstructions and small spaces before hitting the filter itself. Look at the surface area of the stock filter. More surface area means better breathing.
You are right that a K&N filter will breath better than the stock filter, but it will do nothing about the other restrictions in the system. My CAI is a large filter with lots of surface area inside and out, notice the end is open as well. then a 6 inch opening at the filter travels through the unrestricted pipe to the 4.5 inch opening delivering air to the throttle body and VIOLA!
This is my simple explanation to what is a complicated situation, the quest for more speed with the same cubic inches.