First, I will admit that I did not read all the previous posts in detail so you question may alredy be answered to your satisfaction.

I use a "basic bivy" although not the REI brand. Bivys are designed to be weatherproof WITHOUT a tarp. The tarp is more of a comfort item - if it really rains, you are stuck in the bivy without any way to even stick your head out to cook. If you can accept that (I do) then the bivy alone is fine. I use a WM Antelope bag - it has the water resistant coating. The bag is rated for 10-degrees. I also have a synthetic 45-degree bag. For coastal hiking where temps range from 55-60 degrees at night I use the synthetic bag. For mountains (Sierra and Wind Rivers) I use the down bag. I simply do not own a mid-temperature rated bag.

There always is some condensation, particularly at the foot. Therefore I would not use this system for a long trip in constantly wet conditions. You really need to have some sunshine to dry the bag out occassionally. On the coast, I take every opportunity to air my bag - when the sun peeks out, I stop and haul out the bag and dry it, even when on the trail.

The only serious condensation problem I had was when it rained hard, then froze and the bivy was coated with a layer of ice. I have stayed essentially dry in the bivy in very heavy downpours.

The Sierra are perfect for bivys - pretty low humidity with short duration thunder storms and minimal creepy-crawlies. I do use a tent during peak mosquito season. You do have to be comfortable with being really "out there".

I see little point in the bivy plus tarp. If I feel I need this set-up I simply use my tent. My bivy only weighs 1 lb 6 oz total. A bivy is not for everyone. You certainly sacrafice some comfort.