Some people find the 450R shocks to be a bit stiff and tall for the 400ex or 250R. But for a heavier rider this can be a good thing. I think they would work quite well with extended a-arms since the 450R is a bit wider than a 400ex. Overall, I believe they would be a good step up from stock, and much cheaper than aftermarket. I use a set of KFX 450R shocks on my 250R with +2 a-arms, and I think they work great.
Yours truly, sincerelly, love always, Josh aka The DirtDemon.