With a mummy bag from a respectable manufacturer, the draft tubes at zippers, the neck collar and the hood all play a big role in the temp rating. Though most of us here use quilts, or bags/mummy bags used as quilts, the higher end bags are often virtually impervious to wind or drafts. They usually have wind proof shells which make wind a non-issue, at least in my experience. But, with highly compressible insulation, wind could get to the point that - even though it is not getting past the shell material - it is compressing the down. At that point it could still be a significant problem.
For example: most of the UL UQs or TQs we use around here. I don't know if the shells of any of them claim to be wind proof. But I think it is safe to say that with most of them, if a strong wind is hitting them. You are going to be a lot colder than other wise. So get out of the wind, learn to use your tarp very skillfully, or both. Or use some sort of wind resistant sock or weather shield.
I have(more than once) been in a few situations of high wind with my HH Super Shelter. Which has a sil-nylon wind/waterproof under cover. Where I could not block the wind with the tiny tarp if I was going to camp in that spot. I and all of my friends thought I was going to be cold but I was fine. More recently, I have used the top cover which is not waterproof, but is apparently fairly wind and water resistant. I have found it to be a pretty big help against wind also.
So however you do it(sheltered spot, adequate well pitched tarp, some sort of weather sock/shield), you really need to block that wind.
BTW, one of those down/ windproof high end bags probably won't help you that much in a hammock. As you will be laying on the down and compressing it flat, so the bags shell and hammock material will be very flat against your back and a big breeze under your hammock is still going to have a bad effect. You need the wind blocked before it hits the hammock, at least on the bottom.
For I consider that the sufferings of this present time are not worthy to be compared with the glory that is to be revealed to us....that the creation itself also will be set free from its slavery to corruption into the freedom of the glory of the children of God. For we know that the whole creation groans and suffers the pains of childbirth together until now.