I'm new to this forum and only just noticed this post and am therefore a bit late in congratulating you for doing such a great job of stating the bleeding obvious. I agree that 4WDs have there place (and I have one as well as the van) but I've owned a lot of vehicles in my time (and, being ex-military, have driven an awful lot more of almost every type) and the T6 is the worst on wet grass out of all of them (and I don't believe it's anything to do with my driving technique).
Somebody else who knows better will probably be along in a minute to correct me but, in the meantime, I'll go out on a limb here and say that's bollox.