Surprised to hear people recommend K&N on here. I thought it was well known among higher end car enthusiasts to avoid auto parts store brands like that, K&N being the most common example.
I have to disagree that "modified cars rarely get better". Almost every car I've owned has gotten faster on the track, lighter, more comfortable, or sounded better with select modifications. I'd say that's objectively better in most cases. OEM's have to cater to a wide audience, so every engineering decision is a compromise between cost, quality, manufacturing, suppliers, etc. If a certain area isn't up to the standards of a certain customer, they have the option to improve on it in the aftermarket.
Of course many modifications you see on the street are absolute nonsense, so I can see how some can come to that conclusion. However, if you know what you're doing to some degree you can absolutely make your car ownership experience better and more enjoyable.