If it's not too late, I too use the Cave 1 with similar microfibre fabric s-bag. In brief:
1. It mightn't be the lightest at 15 oz. c/w 8 ti-stakes & 50' of triptease (stuff sack is unnessary, but it's the hallmark of simplicity and function on the trail.
2. It is possible to stay dry in some pritty nasty conditions above tree-line. Done it several times in hinge-season weather and, after (4) years, never once had a soaker. High levels of hummidiy/dampness will be your constant enemy with down regardless of tarp or tent.
3. I've tried a silnylon & epic bivis for above conditions, and it's fine, but, really unnessary as you suggest. Simply rigging one end low to ground (i.e., one pole only) is more than effective. If I may suggest: sew an 18-20" foot-box into end of tyvek ground sheet and sleep with your head at the low end for those wind/rainy times (you'll do just fine).
4. I think bug pressure is far more of an issue. I drape a swatch of no-see-um netting (from REI) over my upper body, but can't say I recommend it for everyone. Q: If you're starting at Mexican boarder, would you not be pritty trail worthy/toughened buy the time hit the Origon bugs? This system may just work!
To conclude, I've learned alot about going light from thru-hikers. With the time, your skills and confidence grow, and your and wants & "perceived" needs become more modest. KISS and confidence is the key, and I believe you'll really come to appreciate the CAVE or equiv.
P.S. as a further note, check out the stats page for fellow through hikers on TRAIL JOUNRALS, and note how may nights they spent under the stars vs a shelter - It's a commonly reported stat, and I think you'll find the result rather illuminating.
Very best of luck