The gearing on both K5's is 15 - 47.
The amount of bike lengths depends on how long you go for.
In a rough 1/4 mile drag 15-20 bike lengths between to K5 and the XR650R.
Yes, you can make the XR's scream but mine are only doing 57-59 HP.
Like I said, they are only mildly modified because I like mine to last a while.
Case and point, I'm parting out one of my 01 XR's and it looks like brand new on the inside with very little wear.
On the KTM 300, they are very fast and should keep up nicely with a 250 in a straight line drag.
There again the biggest difference people may see is in the condition of the engine, the tuning and the gearing.
Just because a bike starts and runs and feels fine, it may be bleeding HP.
If I missed a question, feel free to let me know.
Just a thought..........