The cat back adds very little hp, it really comes from the DP + tune. The cobb TBE is a nice sounding and not veru loud.....good choice.
I had nearly identical setup running stk airbox and was getting 290hp on a Dynojet w/tune on my 05 STI.
Yhjere's lots of choices on suspension and tons of info, but the key is spec your entire setup first, just like you've done on the power upgrades. Define how you're going to use the car, your driving style, preferences then seach for the entire setup including sway bars, ciol-overs w/valving matched to the spring rate, bracing, bushing upgrades, etc. Since all of this works to gether look for a balanced system....not a piecemeal collection of stuff.