Generally, what is the upper comfort range of a down winter sleeping bag?

The fill power of down varies considerably, and the quality of bag construction may also vary.  Both fill power and construction combine to arrive at comfort ratings, so it is not really possible to generalize.  My preference is for Montbell spiral bags, based on BPL reviews published in the past; however, others have different preferences.  Also, different folks have higher and lower cold tolerances.  The Montbell spirals have served me well, so have had no reason to make changes.  Although my former trekking companion preferred Western Mountaineering products.

One night on top of a Main Range mountain camping on snow in one of my tunnel tents.
We spent the night sleeping on TOP of our quilts.
There was a warm wind blowing all night, and the snow was mostly gone in the morning.
So – if you are too hot, unzip!

Cheers