Hi David,
You could ditch the GPS. I always seem to question why I carry it around all day at the bottom of my pack. Right up to the point, that is, when the clouds come down and I can't see my hand in front of my face. Then suddenly, I'm not quite as sure as I was exactly where I am on the map!
I opt not to carry a flask, too heavy, tea / coffee always cold when I get round to drinking it, or it's leaked all over the contents of my rucsac. I opt for Ti Mug (60g) and a home-made Pepsi-Can stove (small tonic can actually) and cone (75g) and then carry some 30ml or 60ml of Meths in a Nangalene (sp!) bottle. Either premix coffee and sugar in small container or use sachets acquired from hotel rooms and coffee shops. Would also have a lighter or fire-steel, but I carry that anyway. Guess the whole lot weighs in at around 250g.
Concur on points regarding waterproof trousers.
Would probably add some sort of emergency bivi-bag as well. Had cause to use one before, when forced to spend an unplanned extra evening on the mountain. Which would add another 250g - 300g.
You could opt for a 28g eLite from Petzel in place of the head torch. This gives you an emergency whistle as well. (Though the last time I checked I managed to carry three of these with me. (One built into the rucsac chest strap, one on the end of my compass lanyard and one on the head torch!)
I always carry a first-aid kit, wouldn't dream of ditching it, though at over 200g I'm thinking about pairing it back a bit. Perhaps splitting it in to two halfs, one for short walks and a supplementary pack for longer more remote walks.
I also end up with an assorted odd bag of things like a knife, lighter, lip balm, 5m x 2mm cord, button compass, eLite, spare batteries for GPS, fire steel, tinder, foil blanket, pocket tissues, sanitising gel ... This is usually in with my first aid kit and is affectionately referred to as my "Grab Bag". Something, I think I first came across when I was learning to sail.
I reckon walking in only my thermal top (and trousers!), I'll have just over 4Kg on my back before I add food and water. (3Kg if I was ruthless and did without GPS, warm drink and emergency shelter.) Water will add about another 1.5kg and I always take a little more food than I think I'll need. Interestingly, I reckon I only need an additional 2.7kg to have enough kit for a planned, comfortable sleep out.
Like yourself, I don't think this is particularly light, however unless I was walking in an area I knew really well and was never more than an hour from the car I think I would be reluctant to carry less. I'm now focussing on carrying less weight around the belt line rather than worrying about the weight on my back. So far managed to shed over 7Kg of weight and rather than costing me money, it's actually saved me money. Though my local landlord is the poorer for it.