Firstly, I don't think the changeover is worth it - unless you have some fetish for the climate control !
The trip computer is an easy addition - check out Matti's website :
Matti also has documentation on adding the footwell lamps too. As for the BCM - I'm pretty sure that it's exactly the same ! My Futura has similar (if not, the same) BCM as the Fairmont as when I installed rear electric windows, the wiring was already there - you having an XR means you have front elecs yeah ?
The 'top of the line' sound (aka Premium Sound) has nothing to do with the dash - it's a seperate unit(s). Oh yeah, the climate control uses electric servos and not the vacum lines so that + the passenger side airbag would be the only reasons to upgrade to another dash. Tough call - but if you're super keen, perhaps give Herrods a call.