I have 13/50 only my newly-purchased used 2003 KDX220. I'm not particularly experienced but I'll give you my input.
I've just intalled a Trail Tech "Vapor" speedo/tach/water-temp-etc unit and, assuming it's calibrated correctly, it does about 70 mph at around 9,000 RPM (at which point the motor loses the will to breath). I ride it on the road to get to some of my nearby trails so I have the jetting pretty rich (although I might lean it out in the mid-range some as it is very hot in Phoenix right now and splooging pretty severely).
I've been having the inkling that it's geared down a bit lower than I'd otherwise like it. First gear is a real granny gear, and of less utility than I would have expected -- accept for long rocky climbs (then I don't mind it so much).
I've considered gettingin a slightly smaller rear, or even a 14T in the front but I fear that it would take out some of the single-track usefulness of second gear.
I'll have to get a lot more seat/peg time before I can have a good clue what I want to do with the sprockets (or the suspension, for that matter).
Rick