I would have said a bit more than 5 degrees are added by the bivy, but it probably depends on how windy it is and how well sheltered you are. I do know that when cowboy camping on top of the bivy, if it turns cold and windy, climbing into the bivy and zipping up makes a big difference.
I have found that for me the sleeping bag and bivy combo I use is lighter than the heavier bag that I would probably take without the bivy, so the bivy actually saves rather than adds weight for me with my particular gear. At least that is the case for warmish weather where a cold night (15-35 F) is possible/likely. That description seems to match most of the trips I have done.