Brother, I understand the frustration and dismay this can cause because I have been through the same thing. Actually, GD works in mysterious ways. After listening to my cries for fuel consumption, Suzuki guys decided to take me out on a test. They attached an external fuel supply carrying 200 mL of fuel and drove it around the roads at an average speed of 60 km/h. I was given another bike to follow my GD and monitor its speed and distance covered. I was amazed to see my GD covered 10.4 km in 200 mL of fuel which suggested a fuel average of (10.4 X 5 = ) 52 km/L while it used to do only 40 - 42 km/L according to my calculations.
However, after the test, my GD is still doing 40 - 42 km/L according to my calculations, but it has been proved that it actually does 52 km/L. My mind is boggled why the difference. It is still a mystery to me. So my suggestion is to disconnect fuel supply from fuel tank, connect an external fuel source with pre-determined fuel quantity and drive your bike till the last drop of fuel. This can give you the best value of your fuel consumption.
Regarding your top-speed, you need to make your fuel mixture a bit richer. I also have this issue - my GD hardly crosses 95 km/h mark, but that is because I am using a leaner fuel mixture setting for better economy. If you want to get more speed out of your GD, ask the mechanic to open the fuel screw a bit so as to allow sufficient fuel supply at higher speeds. But get this done only after you have carried out the fuel consumption test! This would tell you the accurate fuel average of your GD, and would help you decide whether you want to reduce this value further by making the mixture richer.
I hope this helps.