I've never been overly impressed with the outright braking power of the front brake on my '02 KDX. Although it will make enough power to lock the wheel in the dirt, it takes a pretty good pull on the lever to get there. After riding a GG300 last fall I remember being amazed at the front brake. Once you get used to it, I think a powerful brake is easier to control right at the edge of traction than a weaker one, where you are using so much more lever force.
Either way, I was resigned to not having that kind of power on the KDX. Then I got my son an '03 KDX and wow... while it is not quite as strong as the GG system it is night & day better than mine. What would explain the difference in 2 identical systems? I have bled until there is no blood left in the system. It improved the lever firmness, but really did not give much more power. Replaced the pads with DP pads, no noticeable improvement.
Could a rebuild of the M/C make a difference? That is where the hydraulic force is created, so perhaps there is a bit of leakage past the seals?