Last year I drove around Morocco in my fully loaded 80 (wife+3 kids+2 roof tents, etc) on 285/75s with some friends with a similarly loaded 80 (wife+2 kids+1 large roof tent) on 235/85s - both sets of tyres were BFG ATs.
In most circumstances the 235s were the better size.
In sand I was having to air down the 285s a lot more than Lio with the 235s and even having aired down a lot his 80 was straining a lot less.
On loose gravel mountain and desert roads the 235s also were better - there were a few times when we were pushing on a bit and I found the 285s lead to more drifting than the 235s.
I have also seen 235s outperform 285s on sllippery muddy fields that we have in the UK.
235 is certainly the tyre of choice for overlanders going from the UK round Africa in 80s.
Having said all that I would probably split the diffference and go for the 255s on a loaded 100