For what it's worth. I've been running 255/85r16's on 6x16 rims for more then 100.000km now.
I have this theory that because the rim is narrower, that tyre is seated better on the rim. And will not run off as quickly when running low pressures (sand driving). But that's just my little theory :-)
I am looking at new rims as well, but because of offset reasons. I'd like to have a bit wider track and clear my chassis a bit more.