Wearing insulated clothing to extend a sleeping bag's range is a good idea because you also have the clothing to wear on frosty mornings around camp or if the temps drop unexpectedly during the day. "Dual purpose" is one of the keys to UL backpacking.
I used a jacket and pants "suit" of Thermolite Micro and medium weight poly long johns in my 30 F. WM Megalite down bag in 15 F. on a windy night in my breezy TT Moment and was just fine. This was at 11,00 ft. at Arapaho Pass in Colorado's Indian Peaks Range.
I think I could have gone down to 5 F. and still been comfortable if I would have had a warmer mattress. The key was the greater girth of the Megalite – hence the name. The girth allowed me enough room to wear the insulated clothes without compressing the down on top.
Now that I've recently had the Megalite overfilled by WM (and it's really lofty now) I think it has become a 20 F. bag so I could go even colder with the insulated clothes, probably to 0 F. max with the right mattress.