Perhaps the issue of pro editing vs. amateur OOTB friendliness could be addresses by having COMPUTER controlled in-depth editing, while the arranger itself stays fairly simple?

All that is needed is a file format that would allow the computer to read the styles, Performances, Voice data, Sampler data, etc., and then you could get into the nitty gritty without making the arranger itself over-complex.

I am pretty sure that most pro's that wanted all this degree of control would be willing to pony up a couple of hundred dollars or so for it, so there would be no need to raise the base price of the Tyros for features few would use.

And, let's face it... SOMEWHERE there must already be these kind of software tools floating around, because SOMEONE designs the voices, creates the Styles, edits the sounds, and does all the detail work YOU can't currently access. Converting it for public access would not be half the task of creating it from scratch.