1. Plumb your blow off valve back into your intake pipe. If your BOV can't be plumbed back then I would suggest you put the Standard Blow Off Valve back on the car.
2. Get a new CAT fitted. Sounds like your one is stuffed or you don't have one at all. Carline Mufflers in Gladesville can supply you a very nice, proper high flow CAT for around $500. Well worth the money.
3. Get your O2 Sensor & Injectors Cleaned. Grey Imports in Annandale is good for this.
4. Clean your Air Flow Meter by removing it and spraying the contact inside with Contact Cleaner
5. Take your car to a Nissan dealer with a Consult Computer and get them to clear the self learning memory out of the ECU. I know Hornsby and Haberfield Nissan definately have them, not sure if they all do.
That should make a fair difference for you. However as you have a higher flowing exhaust system fitted to your car it will never completely get rid of the back firing. The only way to completely get rid of it is to put the standard exhaust back on but there's no need to go that far. It will be alot better if you take the steps I've outlined above