The last time I was involved with a similar situation (the show was Disney's Aida--3 keyboard parts), we rigged all three keyboards on different MIDI interfaces to one laptop running Reason, and then had some custom software that one of the keyboardists used to change all 3 keyboards' channel assignments in sequence (each channel in each Reason bus was routed to a different synth/patch). That was the best cost/benefit ratio in -our- situation, but I think you could accomplish something a little less spartan and a lot easier:
Run both the Mac and the P155 into a small stage mixer, which will then feed your amp/speaker setup (this is quite common for keyboardists). When you are using one of the built-in keyboard patches, mute the Mac's mixer channel, and when you are playing something synthesized by the Mac, mute the keyboard's mixer channel.
The simplest way to use GarageBand for this would be to set up all of your different synths in different software instrument tracks, set it up to take input from your MIDI interface to whatever the selected track is, and then just use the up/down arrow keys to essentially patch the MIDI through to your desired synth (by selecting a different active track).
Other pieces of software will probably have different methods of patching, so just make sure you have a good reliable plan in place. Some people would even use a foot switch to change channel assignments or patch routing.
So to sum up, this kind of setup is in fact quite common. You might have fewer pieces of equipment if you found an appropriate synth workstation, but then you'd have to figure out how to use that, which could be quite a bit more complicated than the GarageBand that you're already used to.