The tyre guy is kind-of right but I think the mileage differences would be consistently different, therefore you can still use the TWI has a guide (better than nothing IMO).
The UTQG tyre rating system is a USA thing from their Dept of Transport. AFAIK, Australia doesn't have a similar system. Therefore if a tyre has a 600 rating in USA, then there isn't an AUS TWI rating that supercedes it.
The TWI is only a guide within a particular tyre brand. ie: TWI of 600 on Brand X will last a different amount of time to Brand Y. This is because the testing is done by the manufacturer, not by the DOT, therefore it's easy to get some result differentiation either by design or accident.
Rather than me copy/paste READ THIS
re: legality of imported tyres.
In the past, it was illegal to fit self-imported tyres. You had to get them approved for use in AUS by the AUS DOT. The ADRs (ADR23/02) have been changed in recent years (i don't know when) and they harmonised with the EU (and the US DOT but I can't find the doco) and now the tyre just needs an E in a circle with the approving country number. (that's how I read the ADR). This is why it's been possible to bring in container loads of 2nd hand tyres and sell them legally or bring in parallel imports & sell them legally.
I guess with any of this type of thing, it's up to the individual to do the research & be comfortable with their own decision.
edit: re: warranty. I doubt there'd be any warranty for a self-import but warranty on tyres is lousy anyway.