well, the sti comes with a catless uppipe, so u don't have a cat, but as far as bends or diameter of the pipe goes, it may be worth it, hell if it comes with a flex joint, i would say go for it, if there is no flex joint, then don't...:thumbup:
You're already in that area when you're changing out the header so you might as well change the up pipe. Trust me, the last thing you want is to regret not putting the new up pipe on then having to go back down under your car and changing it out.
If you don't mind me asking, what header did you get?
There are some headers that come with corresponding up pipes and its best to use the provided up pipe. I know with the GTSpec Gen. II headers, you HAVE to use the provided up pipe because they utilize a 3-bolt flange design at the header collector, where it connects to the up pipe.
How much gain will you get from the header mod? Seems like not much... Also are your headers matched length or offset? If you get the headers that arent offset, wont you lose the awesome boxer sound and move to honda buzz saw land?
Are you planning on porting your turbo? The more flow you add to your exhaust system, the more overboost becomes prevalant.