I'm using the Delta Valves on my '99KX250 and I also replaced the mid-valve components by a "check plate" as they recommend.
I'm about 80kg with gear and 1m80 tall. The bike is set with 0.43kgmm fork springs and a 5.4kgmm shock spring. Shock valving is stock. Oil height is set at 90mm (5W weight).
The Delta Valves kit came with 2 optional springs (with different rates) and 1 optional "low flow" base plate.
There is definitely an improvement compared to the stock settings.
I could set the Delta Valves to get rid of the original harshness while still keeping the same stock (but not so good) bottoming resistance. The fork is really plush in the high speed stuff and provides better control in the turns.
However, I could not "tune" the system do match different types of terrains or conditions. I found one "sweet spot", I mean one setting for both low speed and high speed circuits, which I found good for most tracks. For me, that sweet spot is the optional low flow base plate with the heaviest spring and zero pre-load. I found it good for enduro, trail riding and light motocross application. But any attempt to increase bottoming resistance for more agressive motocross (big jumps, landing on flat ground,...) generates more harshness and less control in the turns...