I just installed a set of Xchallenge cartridges with Ohlins .55 springs and 7.5 weight oil at the Ohlins recommended 60mm air gap.
I'm not sure how the Xchallenge gets 40mm more travel since the springs are the same part # as the F800GS and the cartridge bodies are actually 5mm longer then the F800's. the only thing I can think of is the 100lb weight difference and the preload spacer are what shorten the F800's travel. The fork legs extend within a few millimeters of where they did before install and of course the springs need to break in and settle so it should be the same.
I won't be able to ride it today since we are getting 30-40 mph wind gusts but the initial few rides around the block seemed promising and nose dive under braking is now limited and smooth versus the previous quick and jerky
I spent $300~ on this mod, $100 for the cartridges used, $140 for the springs and $40~ for oil and new bottom screw washers and now I have adjustable compression and rebound damping