Bro my GS is about 6 months old and I am getting 41km/l in city. But the mileage can drop to lower figure if we use bigger rear tyre than the stock one. My friend's 2006 GS150 is giving about 38km/l. Brother if you take care of your bike then even after years its mileage will not be bad. The major reason of bad mileage is when the rubber on GS butterfly carburetor rips. Then ur bike starts eating more fuel and may also start missing. So make sure that you do not get tuning from local Honda guys. They open cards and then take highly pressurized air and pump it inside the card. This works well in cleaning the carb on Honda bikes. But for GS the thin and fragile rubber on butterfly diameter of the carb can rip. Replacing the rubber costs around rs.2000 (for genuine one).
I agree about brakes, they are not sufficient for bike of this size. If you can then kindly go for disk brake setup. It will help a lot in braking.