Its funny... I've dragged my buddies on their xr650r's on dry lakes and it's pretty close to the end. I get em off the line and pull a two bike lead until about 80-85. Then they slowly creep up and pass....
That is amazing.
I own two XR650R's and two KX5 (Back up's are nice).
None are stock, yet none are modified to the hilt.
Both K5's pull the XR's out of the hole and up to the high 90's.
Beyond that I could not tell you.
I wonder if your K5 it properly jetted and geared as it can make a world of difference?
If not there may be other issues, then again I could just be special