In reply to The Ex-Engineer:
If you are trying to get to 500g mark for a sleeping bag, that's probably the best way to go. However, it might be very cold even in the Summer*, if we talk about camping high somewhere in Scotland. I used synthetic 550g (total weight) sleeping bag combined with hooped bivy bag with no mat. It was terribly cold at 900m altitude and I promised myself that I shouldn't do that again. I used a book as a sleeping mat and out of about 7 nights I was shivering quite a lot. Two other nights at bothies were proved to be brilliant. The sleeping bag had a 80 g/m2 Hollowfibre, 4-channel fill.
* I had some very cold summer/spring nights in Scotland, though I also had an extraordinary night. The clouds were so low that until 3-4AM I covered myself only in silk liner. I was still sweating as it was too hot! A night before was way much colder.