Yeah the Aus post ones aren't much chop and they're fairly specifically made to be heavy duty for attaching racks to lug around mail on.
Nearly all commercial ebikes I've seen use pathetic little bafang type motors which as stated are hugely gutless.
My pick would be the whisper ones glowworm sells, they're a larger geared motor more like a mac/bmc and while they're still supposedly 200w they'd most likely handle 500w until the cows come home.
The problem with many commercial ebikes, aside from using baby motors is the batteries. They're largely cheap shit 2C lifepo4 that max out at 20 amps (+/- a BMS that also trips at 20-25A) So even if you up the current in the controller the batteries just can't deliver the juice.
If it's for your work though then they're probably going to want to stick with 200w for safety/legal reasons so nearly all of the commercial offerings will be much of a muchness.
How big is the contract though ? The good folks at Hyena Electric Bikes would be happy to oblige if your company will allow themselves to be gouged like Aus post is