Ian, in all fairness, arrangers are getting SO good that for all practical purposes, they can get acceptably close to the real thing. Secondly, trying to accurately reproduce every squeak and noise from a commercial record can get awfully difficult even for a good musician, and whether you are trying to capture those in audio or MIDI, it can still take quite some time. But, OTOH, the time to do it is considerably less expensive in a MIDI studio!

Next is the fact that Yamaha, making these for their arranger line, they have a well understood and pretty standardized soundset to work with. At least for the modern MOTL and TOTL arrangers, you shouldn't have much of a problem. Probably the likely best course is to make them for the S950, and then scale them up (if possible) on the T4. Probably easier than producing them on the T4 and trying to scale down...

But, don't forget, once produced on these systems, there's nothing to stop Yamaha from selling MP3 recordings of these files to those with less capable keyboards (and including with the SMF for the users that might want both options).

I simply feel that the SLIGHT increase in quality that a full audio recording offers doesn't outweigh the significant disadvantages it forces on you.