The Factory "Hemi Hat" , the flat thing you are refering to is chambered on either side before the air can enter the throttle body. This cuts down on the amount of air being forcrd into the engine and can also cause the air to get heated. (I know not much but every little bit eats gas mileage). Also the factory tubing with the 2 bends and ribbed style can cause friction which causes heat. The stock airbox is not a bad setup actually. It is away from the majority of the engine heat but lacks a little volume intake. I did a "homebrew" doing away with the Hemi Hat and using as straight a piece of piping as I could get and got a 1.3 mpg improvement as well as better throttle response.

 

The Mopar CAI will not improve your MPG.
Don't spend $250-409 on one seeking better fuel economy
especially if you buy it with a credit card and it goes on a balance
and may create interest charges for years.

For $400 you can do any of a basket of real modifications that
can potentially raise your MPG and be cost effective:

https://dodgeforum.com/m_447500/tm.htm