noisy exhausts are fine, but avoid large diameter exhausts(i say this to anyone planning on getting a exhaust system), i know with turbochargers there more acceptable, but large diameter pipes don't always help flow dynamics so much as change them, Stainless exhaust with mandrel bends, thats what i'd look for.
Also i would be careful with short shifters, there fine 95% of the time, but if know at least two people (not usually dumbasses either) who blew engines by mis-shifting right after they had installed short shifter kits, one was a RSX Type-S and one was a Mustang GT. I would install shifter bushings before i would think about getting a short shifter, personally i like a bit of throw so long as the ergonomics of the shifter are good.
Also, by discs at each corner, do you mean bigger brakes? If so i would avoid that completely unless you can find evidence that the brake upgrade is somehow lighter then stock (or if your rears are drums, i don't know much about the 08 impreza), usually all bigger brake kits do on streetcars is add to unsprung suspension weight, which seems kind of counterproductive if you going to be investing in WRC1s.