As far as I can tell, a hydrostatic head of 422 means that particular Cuben Fiber, if fashioned into a stuff bag 16.5" tall, it would hold the water if filled and hung up. That sounds pretty waterproof. What I find amusing Roger is given that, you are arguing against it's usefulness in UL packing for tarps, ponchos, raincovers, and many other practical things based on standards that, by the nature of standards, are in excess of what is needed anyway.
I think a more practical way to approach this would be to find out what the minimum fabrics are that do the job we want them to, and then find out what the HH figures are for those. It seems that much of UL packing has to do with risk assessment so I don't understand your resistance to seeing different meanings of what waterproof can be. This is the practical way to go about things. What is showing here is how limited things can be when we accept standards somebody else set up that we do not question.
We need more information about lifetime and degradation of many fabrics. Most of them get wasted by UV alone, and in that sense I'm willing to bet Cuben has done it's homework.