It depends on how bad the factory air box is restricted, on my 5.7 Ram I saw more of a gain by running a 4" pvc pipe from the TB to the factory airbox than when I had the whole K&N setup with a cone filter.

Back to back testing at the 1/4 I picked up .12-.13 and 1mph in the 1/4 with the PVC pipe and .08 with the K&N intake.

My wifes 5.7 Charger saw little to no gain with a CAI also but the factory air box is designed, the well the inlet and outlet are 5-6" and it has a nice radiused lid.

My neighbor has a 06-07 5.4 F150 and picked up almost 3/10 with the K&N but the factory air box has a bunch of baffles and a small filter, it choked down to 2" or so before the filter.