Fwiw, I've got a SnugTop fiberglass shell that I've been able to load up with close to 500# and still take it deep off road, where the bouncing around from the terrain made the weight close to 800# at times. No problems at all. And that's with making my own mounts for the rack (used Yakima raingutter towers that I disassembled and mounted directly to the shell roof, rails running the length of the truck rather than cross-wise).
Oz- where did you find that bed-mount rack? I mean, I've seen plenty of different ones around here that look good, but nothing quite like that one. Also, I've heard from one of the rack shops in the area that the composite bed distorts over time when using a bed-mount rack