Maybe need to be precise with XBMC
there is the "full XBMC" (including ability to just copy a x86 compatible .dll to immediately obtain compatibility)
and the "patially ported XBMC" (where a small number of drivers, but probably H264 for sure, get rewritten for the new targetted platform)
If you love your XBMC because it can play a movie encoded with a format for which only exists x86 dlls which sources are kept secret or/and strong patent gets applied, then be sure you will be stuck to xbox1, and (eventually) pc0 (silent pc dedicated to home theater).
If you love your XBMC for open sources formats, ok, there is a chance on xbox360. But then it may not be called XBMC at all and it's possible that feature already exists.