About to order Borah CF bivy to pair with tarp for JMT/SHR: questions and expectations

You dont need the bivy
For your use its just dead weight
Sounds like you just want to experiment with one to me, but you have a good shelter that doesnt require it.

Set your hexamid up
Cowboy camp nearby on the groundsheet
If it suddenly rains during night (rare) move inside

You havent cited a single reason to carry a bivy

I used a hexamid, no problem. I got rained on an unusual 5 days and 2 nights with sustained rain.

My last Sierra trip in Sept 2013 part of SHR used a Borah Cuben bivy side zip with a Z Hexamid solo plus tarp..I could have skipped the bivy but it did provide a few more degrees of warmth. Bugs were not a problem, and generally I tucked my pad in, and was inside except my face. In 9 days in rained 15 to 20 minutes the entire time…Bugs I suspect this August with lack of snow and rain shouldn't be a problem, particularly up high. Am myself going back in early August, on and off trail in Ansel Adams plan to use similar set up, though replaced the Cuben with borah argon and sil bivy…you could probably use either como, bivy or groundcloth and be fine…Have a great trip Charlie…

I tried a hexamid tarp (no netting or beak)last summer in the Sierra with my borah bivy. The bivy was kind of large for the dimensions of the 1 person hexamid. I was paranoid that if I moved very much I might kick the edge of the bivy outside the shelter inviting water inside the shelter. Never actually happened, but the lack of margin was concerning.

As the bivy means more to me as I cowboy camp all the time unless absolutely necessary and often push my luck with the weather, the hexamid had to go so I sold it and went back to my MLD cuben fiber tarp that I've been using since 2008. I don't think a bivy pairs well with the hexamid even though I really wanted it to work.

However, the idea someone presented of setting up a shelter as a precaution against rain when cowboy camping defeats half the reason many of us cowboy camp. I hate setting a shelter up and will only do so when forced to.

I often leave the bivy unzipped and folded opened with my pillow on top of the netting. If it's hot then I will just pull the bottom of the sleeping bag out of the bivy and lay it on top.

I agree, you don't need it. Wow, we just saved you a boatload of money;)

Thanks for your replies everyone! I've held off on the purchase for the moment. If I dally long enough then that will make my decision for me. Some thoughts on your thoughts (from my theoretical no bivy experience point of view):

1. The idea of setting up my tarp "just in case" is a nonstarter. I set up shelter and use it when weather threatens, otherwise not. Bivy or no bivy won't change that calculation. (Though, as I stated in OP, I thought bivy might provide me a tiny margin of time in the unlikely case of surprise. Or maybe there is no margin gained since I can just roll my cuben bathtub floor over bag while setting up tarp.)
2. The weight factor is really tiny, probably <2 ounce difference between reg-size bivy and bathtub floor. I don't know if anyone can solve the optimization problem here: can the "about 5 degrees warmth" claim for bivy be made up by 2 ounces elsewhere? I guess some of that is wind protection and some is dead-air-space. My bathtub floor can provide some of the former if I roll it over me in an extreme wind situation.
3. Correct assessment that I am looking to "experiment around". No one needs to try to guess my willingness to spend $180. It's an interesting problem to solve although it probably will make little difference either way I go.

And I got my answer that I can just sleep on top of bug netting or even the whole bivy. Thanks!

On the "not pairing well" with Solo Hexamid comment above, does size of bivy factor into this? I'm 5'8". John at Borah suggested shaving a few inches off the regular size.

I had a cuben Borah m50 bivy and experienced really bad condensation. I had a custom one made that had full netting on the body and sections of m50 at the head and feet.

Even after that I bought a SMD net tent that weighs 7.9 oz and i like it much better and its almost the same weight. Net tent allows me to change clothes, read, and hang out much more comfortably.