It was reported to me a while back that the Tyros2 and MotifES Mega voices were NOT interchangeable. There were differences in the way the Mega part of the voice was implemented. I've been on a quest for quite a while to find some way of adding Yamaha 'flava' to my Roland rig, but this one was a no-go.

And given how inflexible the T2's MIDI implementation is, it would surprise me to find out that the MotifXS Rack is any more easily integrated to the T2 than previous racks...