I tried the moose torque spacer on my kids KX-60. It didn't seem to hurt, but I'm not sure it made much of a positive difference either, and it bent the airbox elbow quite a bit as a result of the new position, and made it harder to get to the choke. So all said and done, I would not do it again, and may pull it off and see if he can tell a difference.
Your best bang for the buck is probably simply gearing down, using decent boyesen reeds, making sure the KIPS assembly is all working correctly, and making sure it's jetted right. This will make a KDX pull pretty reasonably well right from the bottom...