GReddy's are nice, but like he said, you'll pay more. You dont need a full catback if all you want is a nice tone, a performance muffler and tip can accomplish that on its own. With the full aftermarket catback u might gain around 2-3 hp on a dyno, and if you're going with headers later like you said it'd be good to combine them with a catback (this way you're opening it up from the engine all the way through to your muffler). Headers and exhaust you'll probably gain 7-10 hp altogether. With intake i'd say 9-12, overall a nice gain. Also, the GReddy catback does have 2.5" piping while the Borla is 2.25" (the old borla's were 2.5").
edit: those hp gain are approximate for a GT, i dont know about i4 gains.