I know i haven't been here long but just reading most posts on here i'm pretty saddened by the fact that any question that isn't directly related to something that could possibly eek out another fathom of horsepower out of the GA16 isn't worth your time or effort and so decide to strike it down and rip the piss!! Just think it's sad...give the guy a break...he just wants to make a bigger impression with his exhaust tone...reasons aside i thought forums were here to help answer questions not judge reasons and motives for doing stuff!
Anyways mate....
If your going to want a nice noise then i'd plan on doing the lot! INduction right the way through to exhaust...
I've got filter, header, decat and 2.1/4 custom exhaust and it makes a lovely sound, really purrs which isnt' too loud that it drives you insane and is stil driveable at low revs but you know when you give it some...really opens up!
If you were to go for anything first off then it's going to be exhaust without a huge silencer on it which is going to make the biggest impression but if you can afford a filter as well then get that on there and you should hear a nice note all round!
From experience...something like the cybercraft filter or the apexi dual funnels sound a bit nicer than the K&N, not sure on the filtration properties of the cybercraft one but the apexi is meant to be awesome!