The majority of 4x4 owners here in the UK carry those GRP boards. We call them waffle boards because they resemble waffle/egg crates. Due to the amount of sticky mud that graces this fair isle, you would be foolish not to carry them in your recovery arsenal.
hoiked mine to Morocco and the Alps but were never needed
due to versitility and use in mud, "waffle boards" all the way.
Scott - are the GRP boards you are testing from a known manufacturer or are they just generic? the stockists here in the UK make all sorts of claims, but they all seem "generic" being made and cut from the same materials/factories. Its a bit of a minefield to know what your getting. my thinking is GRP board is GRP board! wether you lay it as decking in an industrial environment or use it to bridge a gap, its pretty much the same thing.
G