Actually, the KIPS System CAN be adjusted. Moose Racing makes a "power valve tuning kit" for the Kawasaki KDX- it consists of a set of two (2) springs:

One is lighter than stock to allow the power valve to open faster (my favorite)
One is heavier than stock to allow the power valve to open slower (gives more predictable power for tight trails, mud, etc.)

The spring is what controls the centrifugal "governor" which opens and closes the valves. I think I paid about $42.00 for a set for my '91 KDX in 1992. For the money spent, it is quite the power enhancer and is easy to install.

Also, on another note- the KIPS valve is mechanical and is prone to gumming up, especially if you run a rich premix (smoky). It's a good idea to clean it thoroughly every ten rides or so.