I've taken my 40F bag down to 20F inside a tent but I leave most of my layers on. a 40F synthetic bag can get all the way down to 2 pounds. If something happened to your zipper you won't freeze to death but you wouldn't be really comfortable either. It will make a difference if you are sharing a tent too.

That bag is meant for summer but you can probably stretch it a long way if you throw in a silk liner and keep most of your layers on. a 20F bag is a lot more versatile if you are going to be out below 20 degrees at all. I have a 0F bag but it is 5 pounds so it doesn't get out much.

Another thing to consider is that down is useless if it is wet. Synthetics will still insulate if they are damp. Down has to get almost completely dry before it will "loft" up again.