First of all, welcome and congratulations on your new X5!
1) To my knowledge, your info seems correct. Dinan seems like the best tune with warranty. You can try others, but I'd wait.
2) The brake upgrade is solid. I will be doing this soon. You can buy them here:
http://www.getbmwparts.com/partlocat...catalogid=4462
3) I've inquired about upgrading the sound system. You are stuck with what you have ordered from the factory because the sound system cannot be changed from BMW; I've asked around regarding this and even checked with my service rep. So far, BAVSOUND (
http://www.bavsound.com/) is the only company that can swap out the current sound system (plug and play) with a nicer system while maintaining warranty. They're exclusive to BMW and have really solid reviews. I have the standard HIFI 9 speaker system and will be upgrading when it becomes available. I'm almost 99% sure that the Bang&Olufsen (
http://www.bang-olufsen.com/en/car-a...s/bmw/BMW,-,X5) can't be upgraded to from the Harmon Kardon, but since they are both 16 speaker systems, you might be able to see if they can do an install depending on the sizing of the speakers. Ask your service rep or PM wdimagineer for more info.
Good luck!