noob help. I have an s5. Should I.....
 

buy aftermarket cold air intake? Or should I just leave the box on? Would I even have anything to gain by getting an aftermarket intake?

No. buy an exhaust instead.

Yeah that's the word on the street after a little research.....

What do you have?

straight pipes. no cats.

Sure.
You gain some space and (IMO) a cleaner looking engine bay.

Look at a racing beat system, best system you can purchase for your RX-7.

I'm looking at the RB cat back and i'll probably end up buying that.
I was considering a Borla system but everyone's got raging boners over the RB exhaust so I think i'll jump on the wagon.

Will I run into any installation problems with the stock cat, etc?

Shouldn't. Only issues you'll have with it, or any other complete system, is the possibility of boost creep on turbo models. This should only happen when you ditch he stock system and replace it all with a complete free-flowing exhaust.