I'm not sure how it works on the Xrays, but on my 415, if i place 1mm of shims under the rear block on the front, it equates to just over 1deg reduction in castor, ie I go from 4 to 3deg. Personally I just leave it with the reduced castor.
I wouldn't worry too much about masses of changes to the chassis, so don't bother with spring change etc, it's more of a fine tune (but still has a good effect) than say changing springs.
Just make sure you reset your FRONT droop if you do it though, the new arm angle will mean your droop screws are in a different place
HiH
Ed