I'm the opposite, thought the stock seat foam was fine for sit stand sit stand, but on long sit down rides I sunk into it and felt the seat pan (I'm 190lbs). I bought a new molded seat foam from guts racing in the medium density which is like honda crf foam (but taller in the klx application so softer)

 

I've seen a few members on KF with a corbin seat:

http://www.corbin.com/kawasaki/

Not sure how they stand up to the stocker but have to be a little better i would hope. THeir website shows them for the KLX400 and KLR 650 but I know i saw somebody on the form with a corbin in a KLX250. Pretty pricy as well ($380), but maybe worth the savings on the rear pain lol!