The ST1300 is not meant to do 2-up touring on the existing suspension it seems.
I'd wondered about that already on the ST1100 with its 188kg/414lbs max payload... the 196kg/432lbs payload on the 1300 aren't much of an advance...
And at the end it was the reason for the suspension-upgrade on my 1100, adjustable from 100kg/220lbs to 250kg/551lbs payload...
Some data gained on my ST1100:
Rider + Gear: ~100kg/220lbs
pillion + gear: ~95kg/210lbs ("average aviation passenger weight" there ;-))
luggage/pannier: ~15kg/33lbs = 30kg/66lbs
topcase + content: ~12kg/26lbs (I imagine that many exceed that by far...)
tankbag + content: ~10kg/22lbs
total payload:
247kg/545lbs (exceeding the official max by 59kg/130lbs... more if I keep gaining a couple of stones...)
I'd suggest that everyone takes his own empiric measurements and calculations on his fully manned & equipped mount...