For one thing, most changes of sound within a voice are done by velocity cross-SWITCHES, not with cross-fades. Thus, all voices have the same polyphony (usually, just the one or two oscillators). Now, you start stacking stereo voices like crazy, then yes, even a 128 voice keyboard will choke... But I am inclined to think this is more due to some SA type logic rules, rather than a simple polyphony issue. For an entire PART to drop out just because one voice of polyphony is exceeded seems a bit strange... And because it IS those fancy vocal sounds, I think the issue relates to more than just the basic polyphony of the instrument.