I have a 05 Infiniti coupe with OE intake system and I installed k&n air filter.

The majority of aftermarket systems that are out don't really have any housing for the filter. With this in mind, would it be ok to leave off the air induction that sits right under the hood above the radiator and the power duct cover.

 

Leave those parts in place if you're using the stock airbox. They help direct cool air into the intake vs sucking hot air from the engine bay. The Stillen intake retains both of those parts for that reason. The majority of other aftermarket systems suck (no pun intended).