I did do my forks using my left over XChallenge cartridges. They have a spacer to reduce travel to about 245mm. There is no machining, you just change the cartriges.
Very impressed as cost was minimal as I had the cartridges already. I think the XC internals suit the 800GS better than the XChallenge ... great improvement on and offroad. Used my left over Hyperpro springs with a spacer for the extra travel. This is the only fault I notice ... forks top out at full extension a little bit, so I might try different spacers one day.