Initial upgrade for STi
 

Gentlemen I am planning on a basic upgrade;

-New Back Box
-Walbro Fuel Pump
-Panel filter
-Remap

Do I really need a sports cat to achieve around 310 BHP.

Its just in my lack of knowledge I was trying to imitate the Prodrive setup, and thought it might be necessary for the sports cat.

you would get more spool up and better torque by going with either a sports cat or de-cat pipe, the oem cat is pretty restrictive and you would get pretty low figures after a remap.

You should be able to reach 310 with your standard exhaust but like LJF said it will drive much better with a sports cat, or better still a Decat. You could safely aim for 320 with a Decat too!