Yes, a "typical" steel fork does weigh in the 700 gm neighborhood and the stem weight you estimate isn't that far off either. However, a 350 gm carbon fork is pretty high end and 400+ grams is more typical. Headsets typically weigh 120 to 150 grams so you aren't going to save much there.
So for a 400 gm or so weight savings you are going to spend $200 or a lot more to reduce your 10 kg bike to 9.6 Kg for a 4% weight loss. And, if you expect a carbon fork to be a dramatic comfort improvement, you will be very disappointed.