As mentioned by a few others, I would tend to agree that a brand new car (of any sort) under finance is not really a 'money saver' at all. A 'modified' brand new car with a voided factory warranty will depreciate even faster.
I would suggest to the
OP that if the priority is to save money, then an option would be to off-load the V8 and get out of the finance cycle altogether if possible. An older stock standard Falcon (worth say $4-6k?) , owned outright, would be a cheaper proposition than a brand newy. Maintenance costs on an older car?.... still way cheaper than a brand new one's interest and depreciation I would say. Certainly not as snazzy as a new ST (yes, that would be a lot of fun!), but it would save you money.