Some people may differ but my philosophy is that I would rather be hungry than cold. Meaning that I would rather shave weight on food than a sleeping bag. Not sleeping well is exhausting and demoralizing. I don't ascribe to the old infantry "travel light, freeze at night" any more.
Also remember to calculate the weight of whole sleep system. For instance if you go with a light weight bag that isn't as warm you may want to bring along some heavier sleep wear, whereas with a more robust bag you can sleep in your silkies.
I use a heavier bag with bivy but I partly make up for the added weight by being able to use a tarp instead of a tent.
leo