So yes, the car only downloads the differences for a new update. They use a package from a company called RedBend that builds these patch files. They are often pretty small relative to the ~700megs of software in an install. 8.0's patch files for most people are probably going to be under 100megs.
The CID then depackages the update and builds the full install and verifies it's got the correct signature, and only then do you get the notification.
I will try to post here as soon as I see the first real 8.0 hit any cars so you guys can start stroking your rabbit feet. =)
BTW, I'm on a long road trip testing out the new AP. It gave me a false FCW event earlier today and the AP recorded the 8 image EDR sequence to upload to the mothership: