That is a great question.
I can't wait to see the answers from other members.
I do not have any HP figures but i have owned and rode nearly every year of KX500.
So, what I am saying is these are my impressions and based on basically stock bikes.
When you hit the pipe I feel that the 89-04 are all the same and seem to be faster than the 85-88.
The 85 felt the least powerful.
If you own a 85 don't squeal, it's only my impression.
86-88 all felt the same and fall in the middle.
There again it is only my impression.
On the 89-04 what seems to be the biggest factor is the state of the engine.
How good is the top end, seals, jetting, ect.
So, who really knows the real facts, I'm all ears