I have a '08 250 sx, The suspension has been done by a highly reputable suspension company here in Australia, and they have done wonders with the front end, but there advice was to stick with a straight rate spring on the rear.
It works great under power, but under a trailing throttle at speed, get ready for anything.
My thoughts were that a progressive spring would deal with this situation better, as the suspension is unloaded and would be using the "light" part of the progressive coil to absorb the hits, instead of the stiff part of the straight rate spring.