I think it's very hard to use one bag for such wide-ranging conditions.
So, I use a 2-bag system which gives much versatility, and only pays a weight penalty at the coldest conditions when needing to pack both bags.
I have a WM Alpinlite and a MontBell UL Thermal Sheet Spiral down hugger.
For summer conditions above 50*F, I use the MontBell alone, and it's very light at ~13.5 ounces.
For conditions below 50*F but not much below 20*F, I use the WM Alpinlite alone, at just under 2 pounds.
If I'm going into real winter, and I might need to get 0*F comfort, I bring them both, and put the MontBell inside the Alpinlite, and it's nice and cozy. Or, if you wanted to, you could use it as a quilt on top of the Alpinlite. Whatever you wish.
Yes, having the two bags does cause a small weight penalty when carrying both if it's really going to be cold, because there are two shells involved. But in the summer you get a great weight reduction with just the MontBell, and you have a normal weight load in the regular conditions with the Alpinlite.
I find it to be a very good compromise, which has at least as much benefit as drawback, and since I do more camping in warmer temps than really cold, it works out great for me.
It does cost more to buy the two bag system, though.