There's a few options with everything you're interested in, so it pays to do your research on how you want your car to behave, rather than what everyone else wants.
For instance, there are OTRs that will require a MAFless tune (Russo, Ramjet, etc) but there or some which will allow for a MAF tune (ie, retain the MAF sensor for legality reasons like the VCM OTR intake - although this one can also be MAFless). The difference between MAF and MAFless is not such a big issue with the VEs - it used to be a significant restriction but now there's around a ~1-2rwkw difference, depending on your tuner. Some still prefer to remove the MAF while others are much more willing to retain it.
There's also a variety of headers, cats and exhaust designs which can make a difference. There are pros and cons for long headers vs short, tri-y headers, varying degrees of high-flow cats and varying degrees of legality.
There are a few places in Sydney that are definitely worth talking to. Oztrack is one, also Marrano's in croydon, SAS in silverwater and Castle Hill Performance Centre. There are plenty of other quality workshops but these would be the 'famous' ones. I know Cromer Exhaust does some very nice work, as does Northmead Performance.
Unless you're planning on taking the FI route or camming the car, then there's no NEED to go bigger than a 2.5inch exhaust. That size (depending on design) is ideal for a mechanically stock motor - however you won't find any negative effects going to a 3 inch - some systems sound better as 2.5, others sound better as 3 - so spend some time on youtube, having a listen to what other people have fitted (and if the products are not available locally, it's easy to order interstate and have it shipped to your preferred workshop) - but also call around some of your local exhaust shops and go in and have a listen and see what you like.
OTR, full system and tune can cost anywhere from $3,000 to around $9,000 - depending on brands and/or custom work.
