I had made an all 1443r tyvek double bivy for my wife and i to use. I found it tore and frayed somewhat easily and i didn't sew it, but used tyvek tape which works really well for this and most other tyvek materials. After about two or three times of use with my wife, wherein she wasn't as careful with it as i was (and she would sit up fast or what not stretching the material), it showed clear degradation. It wasn't the bottom, i was still using 'polycryo' underneath (and the bivy often on top of our pads)–damage occurred where it was getting stretched.
It just doesn't have much strength and durability compared to silnylon. On average and as a trend, a well and even decently made silnylon tarp should last much, much longer and more functionally than the "kite" tyvek.
What i would be personally interested in is hybrids of polycro and the super light nylons like Titanium Goats "nobel" (because it's relatively inexpensive). 30D silnylon will be more durable longer term, but combining polycryo with a very light nylon (if it's feasible, i haven't tried it yet myself) would create a more double walled design further decreasing condensation, decreasing true misting, increasing insulation a little all at a weight slightly under silnylon (provided you use a lighter/thinner polycryo).
But, the problem with this design is one that would be a problem with the kite tyvek as well (but the tyvek has more problems over all), if anything fell on it from above, it would easily pierce the polycryo if that was on top (which during rain, you would want the polycryo on top).
Again, i'm not sure it would be really feasible. My thought is that if you rolled/folded the nylon over the polycryo a few times or so at the edges, and then sewed the edges, it might hold the polycryo in ok, but then again it might require tape or adhesive. If that did work, the tightly woven nylon would provide the load bearing strength, and the polycryo the waterproofness (and again the double walled benefits of the combo). If i ever get around to this project, will let people know how it goes.
In reality though, a 30D silnylon tarp would be cheaper and more durable than this option, so in some ways more efficient and better.