3" is way to big for a NA 2.0 litre, I've got a 2 1/2" exhaust system on my 2.5 litre and I wouldn't go any bigger. If you're going to keep the car NA, get a hold of some extractors (there's a group buy set up at the moment, to get Cobe's for around the $300 mark), and if you're going to slap on a pod filter, make sure you sheild it so it doesn't suck in the hot air from around the engine.
At the end of the day though, you're pretty limited with what you can get out of the 2.0 litre engine.