I had one of the cheaper ones (MSR) I think, it just changed the leverage (traded a long throw for less effort) and I never liked the looong throw. I don't know anything about KDX clutch springs, but haven't heard anyone bring them up, so they don't seem to be causing problems. So, yours may not be stock. The Barnett clutch kits I have used all have much stiffer springs, required much more effort. You might pull your springs, (for a size reference), and go buy some 125 springs. First replace every other one, if still stiff replace them all.
I had a rock hard pull one my DR. I changed to a motion pro terminator cable (way better than stock) and Then after a week I added a Applied racing quick adjust perch. It has a ball bearing lever and different angel of pull and a set screw to adjust the lever position and it works great. Mucho improvement over the old set up. If you do that route just make sure to loctite the set screw on it. It backed out on mine actually partialy enguageing the clutch and causing slipping. Too a minute or to sort it all out, but over all I'm happy with it.
I recently tried the MSR Raptor system on my '03 KX250 and didn't like it. Like OKKX'er says, it just changes to a long throw. Did not help with my arm pump at all. Plus, the levers are cast aluminum and do not survive very well in tip-overs that involve rocks. And you have to get the same kind of Raptor clutch lever as a replacement (stock lever won't work). If you want to go higher-dollar, I've read that the Hebo hydraulic clutch is generally easier pull than cable clutches. But it will set you back a couple bones.
It has a wide/long stock throw as is, so maybe a change in throw rate will lead to premature clutch wear and/or engagement/disengagement woes, meaning the Easy-Cluch is out for sure.
It's not unbearable but noticably heavier pull than my smooth 250's. I wouldn't want to live with it the way it is but it'll do for the kids, though having something easier to work with would encourage them to use it correctly rather than avoid it altogether.
I think one of you is on to someting with the springs - they may very well be aftermarket and therefore will be replaced with OEM upon rebuild.
(I was going to post in the KDX Fourm to see if this higher pull is common but figured the post would get moved here.)
Although a Hebo juicer is tempting I really should stop dumping more high $ stuff into this bike, unless it one day become my main ride; but then again there isn't much else to do the thing.
Dan: my DR had similar buttery-smooth action as my RMX, but I believe that changed when I put HD springs in with the 441cc kit.
Switch out the OEM two-cable device to a Motion Pro CR throttle tube and housing. I cannot remember if I used their cable or pull-side OEM cable, but in the end it's the same setup Stroker used for the KLX with a Mikuni pumper carb; no push close cable is really needed.
The lessening of pull-tension is welcomed, takes a few minutes to get used to (whoops!)