I am considering changing just the downpipe and the catback exhaust on my completely stock FD R1. With those mods alone will I run lean?
Do I need an air fuel meter?
Do I need a fuel computer?
I'd like to keep my car as stock as possible.
Thanks!

 

If those are the only performance mods...no. There was a well established "three mod rule" years ago that IMO is still valid for a stock car. You can search in the 3rd gen section for more information. It all really boils down to maintaining 10 psi. If you do that, the stock ECU is sufficient.
On a side note, if your car is truly still 'stock', I still recommend an aftermarket temperature and boost gauge...so you can monitor those critical things. Along with reliability mods such as all aluminum radiator and AST.

 

Like Sgtblue said, you'll be fine. Especially if you're still on the factory air box and catalytic converter.