I don't like them. Too harsh and just some wrong feeling. I have used only 10 mm spacers with factory spec for air gap and oil. Maybe I had to try lower oil level to compensate for volume of the spacers. My 2007 XTR came with progressive springs from the factory.
For good and smooth asphalt they are OK. I am only 60-65 kg., maybe 70 with the gear. For heavier riders... I don't know.