You could put a solid block of wood between the axle and chassis on any car and it will still go down at the back and up at the front if you put too much weight too far back behind the axle. You must have a hell of a lot of heavy things in the back if you think the heavy duty model will still be not good enough.
Whether something breaks or not will depend a lot on where you drive it and at what speed. Swaggie said he has been driving in the VHC for years and has not broken anything. There is no speed involved in those conditions. I have seen many people on forums ask about suitable cruising speeds on Outback roads but never in the mountains for obvious reasons.
Whatever obstacle you hit on the road be it a rock ledge on a bush track or a hole at a cattle grid will send a certain amount of force up into the car. Those forces increase by the square of the speed. A big hole at the approach to a cattle grid or a deep washout will send up forces sixteen times higher if you hit them at 80 kph than at 20 kph. That is what breaks so many overloaded or incorrectly loaded cars in the Outback.
I think far too many owners are lulled into a false sense of security by aftermarket suspensions, big wheels etc that come with all kinds of must have advertising. They head of into the Outback thinking they have a built a "tough tourer" that is indestructible.
I hear you mate. I still disagree with the factory suspension being suitable. I'll get it all weighed when the build is done and we will know for sure.