towing with new fangled auto / change rear springs

Not that I think airbags are really suited to the lighter chassis of modern vehicles (maybe ok with small pressures as assist) but the new BT50 would be able to tolerate the airbags better than the older weaker chassis on previous BT50's.
It worked Ok but the new one is stronger in the rear rails sections.

Just an observation will reveal that fact.

If you run the engine revs at the road speed you wish to travel at so the revs are at a point where the power and torque curves just about cross each other it should be the most economical at that position without loading the torque converter, which if it has to slip will create fluid heat above what is necessary.

Towing! does it have a transmission cooler, so IT gets rid of the auto fluid heat before dumping the heat into the engine cooling system water? That keeps all systems not being heat stressed.

M&N's,
Have a look in your operating manual. When towing pull lever over into sports mode and let it do the rest. It will change down earlier and also not change up if load is too great. The vehicle will also tend to rev out a little further if a load is sensed, before changing up.

It also probably won't drop into 6th gear below 100kph if it drops into that gear at all.

As for the springs I don't know if I would go to 500kg unless they are very progressive springs. See if you can go for a ride in one empty fitted with those springs.

For what it is worth, my vehicle has 300kg OME springs and tows 2.5t+ with the vehicle weight around 3050kg.

I also am not a fan of airbags.