I haven't been on here for a while, but if you are still interested, I would say there isn't too much of a noticable difference. The main reason why I did this was because I personally prefer the mirage with the oem spoiler, rather than it not and lookin like a ball .More interested in if OP(or anyone else who has added one) can give us a real world feedback on the MPG change instead of comments from the peanut gallery...
As Mark has said, yea in Australia we don't/didn't get spoilers from factory if you have the ES model (Maybe SE as well?), Only GT model has it straight from factory.Only someone from a different market (without a factory rear spoiler) like Australia could comment. That's if they added one.