I did take a long hard look at this. There's some money in it for someone! The throttle action is just too long and it makes the bike feel much slower. My KTM Adv has a much quicker throttle action and is far more fun as a result.
Back to the GS - I nearly started to work on this one but in the end it was easier to flog the bike. The best option is to replace the plastic moulding which the throttle cable at the grip-end goes around. Making this moulding larger diameter would achieve what you want without having to mess with the actual throttle cams.
Initially, I was thinking of something which you could just fit/glue on top of the existing moulding, but decided this wasn't a good idea as it could come off and jam. Next step was to be to take the existing moulding off, get it scanned, and get some larger diameter ones made up by a plastics fabrication shop. I didn't get this far as it would have involved paying up some cash.
Putting new throttle cams on the throttle bodies looked far too fiddly.
The moulding in question is easily seen by undoing one screw underneath the throttle grip housing. This pops the top off the cover so you can see it. It looks like you can disassemble the white moulding bit quite easily.
Have fun!!