Thank you Pixyflash, and to the PMs I've had regarding this.
I've been back and fourth with the Subaru Technical IMGroup and they've come up with some great information and a progession step.
For future reference if anyone is ever in the same situation, looking to upgrade the media unit to the 7" Starlink infotainment system, Subaru have patch looms available through their accessories dept for the Starlink and for Pioneer units.
The only downside for the Starlink unit is that the reversing camera wiring is not in place and so that feature is not available.
If anyone in future needs the wiring diagrams for all three media systems then I have them from Subaru directly.