2.5" catback provides a nice note, not really much gain engine wise altough if you got a set of extractors as well it would make a bigger difference.
they only cost $250-300.
I have installed extractors and 2.5" cat-back system recently. Attached is the link. Installation of the cat-back is simple - a 20 minute job to someone with a bit of mechanical know-how. The extractors have made the difference with the sound and I reckon there is more torque now but hard to say.