I used the K&N flat filter in my stock air box. I didn't like the idea of sucking hot engine bay air into the "Cold" air intake... The stock box gets it's air from outside of the engine bay above the fender liner.
If your aftermarket intake doesn't involve cutting a hole in your fender, it's not sucking any cold air. Jokes asaide, these "CAI" systems have been proven bogus as far back as the import tuner trends in the early '00s. It's just the 21st century version of "ram air". If you just want it for the look, that's totally fine. To each their own. Just don't expect it to do anything for performance other than potentially lowering it.
No, no difference. lol, you won't get that cool CAI sound with the stock air box. After I added a snorkel, I noticed the cold air intake sound. And that is about as Cold Air that you can get. lol
Snake oil. You already have a cold air intake with plenty of flow unless you've added some serious horsepower. Very few 21st century cars, especially recent ones, benefit from altered intakes with the engine in stock form. So much engineering and research have gone into optimizing intake systems in the past 20 years.