I decided to try putting together a jazz trio as a Performance. So I added drums as Part 1, bass as Part 2, and the Bosendorfer Imperial Grand as Parts 3-5.

I turned Zone Master on, and limited Parts 1 and 2 to the lower half of the keyboard, Parts 3-5 to the upper half of the keyboard. Then I entered an octave shift of -1 for each of the three Parts of the Bosendorfer Imperial Grand.

Things got confusing at this point. The boundary between the lower and upper halves of the keyboard shifted rightwards, for one thing. Other unexpected things also happened.

Am I going about this the right way?

No.

Is octave shifting the Bosendorfer Imperial Grand a practical thing to try to do?

Yes.

You are going about it in a curious way. If all of the sounds you are playing are "local" to Montage, there is certainly no reason to use the Master Zone function to accomplish what you are doing. It's not wrong, just not necessary.

We'll answer first why possibly you are getting the strange behavior... Each PART has a Master Zone setup. You need to transmit (your way) to each Part internally via a zone. (This is totally unnecessary - and quite different from playing normally. Simply place the Parts you want to control under KEYBOARD CONTROL - the green icon on the main Home screen).

"Octave Shift" is a TRANSPOSE function that deals with transmitting. This is what Master Zone function uses to change keys. It is a function of the keys transmitting.
"Note Shift" is a TONE GENERATOR function that deals with receiving. This is what the synth engine's Part parameters use to change keys. It is a function of the synth engine receiving.

This is true (even in the XF) "Octave Shift" +1 moves your split point. "Note Shift" +12 simply changes how the tone generator sounds and leaves your physical split point where you physically want it. (Use the words interchangeably at your own peril) 🙂

There is no need to invoke the Master mode, you click the "+" icon to ADD new Parts (excellent) - you can have up to eight for realtime play - your Performance has five Parts, simply activate "KbdCtrl" on all five Parts on the HOME screen (green); Leave the ZONE MASTER switch OFF; use parameters within each Part's own architecture to "Note Shift" and "Note Limit" your bass Part and your three piano Parts, place the drums across all keys but place it under Arp "Hold" so it plays continuously and "Arp Play Only" so it is not heard when you press stop.

There is no need to invoke the Master Zone functions to address itself in your particular goal. It does a much better job controlling itself directly using the KEYBOARD CONTROL function available per Part 1-8.

I can imagine you can simply multiply your confusion trying to address Montage engine through both Keyboard Control + Master Zone... Your using two tools on the job, one will do. The best tool for what you are doing is just use the internal KEYBOARD CONTROL feature and the regular normal Part parameters for your purpose.

Hope that helps.