Yep. Plus composit decking needs minimum 3 supports. I'm looking to span 15 and 20 inch spans between rails. But your right, at 8" wide this decking will probably lose it's strength if used as a side deck.
Another use for the attic deck I just thought of is the front decking over a bay. Line them side by side and cut to the width you need. Then run a couple tubing runners lengthwise to tie them together attach with some stainless steel hardware. Keep the runners inside the frame rails and let the decking rest directly on the frame cross rails, just like the product was designed to do. Boom. You have a custom platform to your specs. Super lightweight, no staining, self draining, easy to run straps through, more stout then polymax, for $50-60 bucks. Add some table legs that stow flush inside the runners and you'd have a nice little river table.
I like what some of you guys have done with polymax, but have been looking for something more sturdy. This might be a good alternative.