In my experience, bivvying in any temperature above 50º is for masochists and others who thrive on suffering. Even bivvying sub-freezing is not a thrilling experience, but at least doable. I have an OR Alpine Bivy and an REI Minimalist Bivy.
If you think a rain jacket doesn't "breathe" well enough when you are walking, just imagine how well the same material will "breathe" when you zip your head in there and exhale a liter of water overnight? When you sleep your body loses about a liter of water, which will go... right into your sleeping bag and bivy.
Once the weather goes to freezing temps, expect to wake up with icicles growing above your head. Not the worst, but an experience nonetheless. Sometimes it is too cold to stick your nose out of your bivy, and you are gonna zip yourself in. Icicles result.
That said, I think you can expect about 5º of warmth from a bivy. It certainly blocks the wind and certainly keeps in heat. However, keep in mind that you will get some amount of moisture build-up inside your sleeping bag, which will compromise your bag's loft or ability to retain heat. I try not to recommend a bivy to most people, but it is certainly a workable shelter and something I usually consider taking when the weather gets cooler.