last time i used xsi ( 4.2 ), the displacements in xsi were rather horrid to use.
the setting up of a shader to give you accurate results to what you had in zbrush usually needed 2 math nodes/ lots of tweaking. then sometimes it wouldnt like to read tiffs if you chose certain settings wherever you saved the tiff out of. sometimes it would just crash because mray felt like complaining.
then there was the bug where if you had subdivided your model once in the viewport, xsi would take that as 1 subdivision+ whatever you had in your subdivide @ render time settings, which would crash xsi entirely ( not just mray ).
so we had to resort to using the map format, which was a pain to convert to every single time ( especially when your doing tests ).
in renderman you just load in a map and press render. theres really not much else to it.
so anyway, if xsi has a mental ray version that has fixed its displacements ( because it has been an area that mental ray hasnt been really fixed at in a while, then i dont have much hope for it in terms of displacing geometry.
it can do it, but its a pain.