My KX forks came with the caliper and everything. So I stuck with the KX caliper and didn't even try the KDX caliper. As for the springs in the KX forks. I am 170lbs. and ride both MX and woods. Spent Saturday all day in the woods and forks where a dream. Spent all day Sunday at Cooperland MX track and loved that too.
The stability and handling over ruff ground is awesome. I do, however, run a steering damper, so that does help. But in comparison to the traditional KDX forks. I would do it again in a heartbeat.