That you've booked WT to start with suggests that value is important to you, and paying double for a 'bit more' is difficult to recommend. WT+ is no CW.

You'll be knackered(!) when you arrive in SYD, but it's not the case you've got to head to a meeting - a day lying on the beach or the botanical gardens will sort you out.

Having done the LHR-MEL 'straight through' my recommendation would be get pi$$ed to SIN and sleep to SYD - you'll at least be waking up at the right(ish) time.