I've looked at the cuben/nylon laminate, just under 3 oz, and one of the very light Xpacs around 3 oz, and did not like the stiff, crinkly quality of the fabric. Was also concerned with how durable the Xpac would be, with the very thin fabric portion of the laminate, and how well stitch lines on the cuben would hold.
Also looked at 1.9 oz silnylon, available from Seattle Fabrics, weighing around 2.5 oz. with coating. Unlike the stuff Warmlite uses for stuffbags, it has a double coating, and is therefore a little more waterproof, but not so much as Xpac.
In the end, decided on the 4.9 oz Xpac purchased form Rockywoods (VX07, I think). It is much more waterproof than a couple of dyneema/spectra gridstops I looked at, and i think it will be just as durable. The weight penalty for a 4.9 oz over a 2.5-3 oz material will be 2-2.5 oz for a pack using around one square yard or less of material for the outer fabric.
For me, 2-2.5 oz. is a cheap price to pay for a pack that will remain waterproof, hold up to abrasion and wear and tear, and last for many seasons. I made a pack of 1.9 oz ripstop nylon with a stormproof coating that brought it up to close to 4 oz., and while totally waterproof, it started to show serious abrasion after only one 20 day trek - in high country, not bushwacking. Not going there again.