Honestly, I have thought that through some, and have used the i-drive a bit to know the difference. I really do enjoy the ease of the Garmin NUVI GPS and finding POI's, etc. I just HATE pulling out my NUVI seemingly everywhere I park in fear that my car will get boosted for the more than obvious GPS mounted on the dash...once in my car, they'll grab the ipod and who knows what else.
I also like the other functionality in the i-drive (besides the GPS.) Controlling my ipod through the Professional radio right now is stupid & ridiculous - I may have to fashion the USB retrofit (6FL) at the same time.
I suppose once the dash is in, it should be easy (assuming right now that the instrument harness is the same) to go back to my original configuration...except it'll require a custom bezel to remount the Professional radio (as it loses it's mounting location with the double-scoop dashboard.)