My westy frequently has to haul around a variety of heavy and/or
bulky stuff. How about:

80 cu ft (20 x 4 cu ft sacks) of compost.

All the insulation for my attic.

20 wall insulation bags + insulation blower.

The heaviest I ever carried was probably when I bought a full set of
flat roofing stuff from a roofer on an auction. The van got about
half full with the real heavy stuff, (asphalt, glue, roofing felt...)
and was topped off to the ceiling line with 4' by 4' high density fiber 
boards. This was before I started to inflate to 50 psi, and at the
drive home I was afraid I was going to wreck the tires. No problem
with braking or suspension though.

Contrast this to the american and japanese stationwagons I've
used for similar purposes. Typically the suspension would bottom
out completely, and braking distanse increase significantly if
I filled them up with similar things. This happened even with one
I had equipped with HD suspension and towing package.

/Martin