Funny, your post reads a little like one I put up a couple years ago after looking at a sample of the 1/3 ounce version of cuben Quest Outfitters sent me.
You might want to take a look at Richard Nisley's fairly recent long threads on the MYOG and GEAR forums on this site, wherein he tested many samples of cuben and silnylon for water resistance.
Since then he has posted more than once that cuben fiber material with the .18 suffix has more substantial and water resistant layers of mylar (the stuff that looks like cellophane) encapsulating the fibers than does the material with the .08 suffix. If you can obtain cuben of the desired weight with the .18 suffix, you will, if nothing else, feel better about the durability of your equipment. After reading Richard's threads, you can draw your own conclusions about actual durability and water resistance.
A number of us purchased cuben prior to Richard's reports, but are making the best use of cuben with the thinner mylar. It's way too expensive for many to discard, and is as water resistant as a lot (but not all) of the silnylon currently available; and in the opinion of many is quite serviceable, and clearly, does not sag like thin silnylon when its surface temperature drops (note posts above). Forgive me, but this topic has been raised on so many threads here, that I have become too lazy to look up the links yet once again. But hope the above will help to get you started if you want to delve into the subject. Just use the BPL search feature.
FYI, my own decision, adopting the suggestion of another poster, was to use my 3/4 ounce cuben with the .08 suffix for tent end covers (the portion of the canopy that covers the vestibule(s), but not the sleeping area) of a tent. (This raises the sticky question of how to best join cuben to other materials.) Many others have reported quite satisfactory results with cuben tarps made entirely of the thinner mylar cuben, with reinforcements at stress areas as found on most backpacking shelters. As of yet, I have not completed the project and put the end covers to frightful weather, so have no strong opinions either way. I am confident enough about the material, however, to put in the hours making the tent. Ironically, from all reports, it is made of a better quality (but not thicker) mylar material that may have been sent to me through an oversight. Time will tell, and hopefully, BPL and I will still be here to post the results.