Planning to install an aftermarket catback exhaust but not too sure if it will cause any problems with reaching the maximum noise level.
People are saying to install two mufflers, will this reduce the sound?

What is the maximum noise level in Melbourne anyway?

Also if I was to install it myself, are there any things I will need to change, such as the cpu?

Thanks guys!

I find the Xforce system in my BA pretty good, not too loud but a nice deep tone on idle and a good sound when you put your foot down with very little drone while cruising. I have 2 mufflers and am happy with this set up but mine also has extractors.
The only thing u gotta worry about with the BF is the o2 sensor after the cat as the increased flow may cause the engine light to come on but this can be fixed with tuning depending on how much cash you want to throw at it.
I had mine done at Auto Excess in Coburg who basically told me that no aftermarket system is legal if it exceeds factory decibels.
Good luck with it mate, that's where my mods started and continued until the car became my gf's...