you are making some really big assumptions here about an aircraft being able to be flown overweight in other countries. They may not have the same G requirements as Australia, I believe, our G requirements are based on international standards plus the weather that can be expected in Australia to affect an aircraft during flight.
Just because somebody says they fly at 700 kg overseas is probably true but it will most certainly be at a reduced G loading so instead of +4-2 you could be +2-1 in which case your aircraft will either be damaged or break in our turbulence.
There are standards and rules for a reason, commonly these are written in blood of those before you.
CASA or RA-Aus has a requirement of a required payload which is one hour of fuel, two persons of 86 kg each and if you add this together and don't have enough payload based on the aircraft empty weight then the aircraft cannot be registered as an LSA and would have to go into some sort of experimental category with CASA. We saw this happen at our airport previously when the RA-Aus came up to measure some aircraft that were being assembled that were obviously much heavier than being claimed, then all of a sudden they went on to the VH register after the RA stickers were removed