mjh_056....
Only had the car a few days so no time for any real testing, but here are my initial thoughts anyway in answer to your questions
1. It is quicker (I think) but not by much as I already came from a Nav Pro so that already had the faster processor and more memory than the smaller unit, but yes, it does feel a little snappier (especially when switching from one screen to the next). Start up times from cold seem about the same.
2. UI wise, there are a few. Fonts are different (and slightly smaller) on the Media screens, allowing more of the track title/artist to be shown and just looks a bit neater. Radio wise, the DAB and FM logos are now automatically downloaded over the air and show correctly in full size when on the station information screen (this never worked on MIB I). The "CAR" menus now has additional "Think Blue" screen (naff), and I have coded in the Off Road screen (in the absence so far of the performance screen that the MY17 cars are getting).
Also, one big and welcome change is that rather than pressing the CAR button then having to press the small menu button in the lower left hand side of the screen to switch between screens (car status, journey stats etc.) you can now just keep pressing the CAR button and it will cycle through all the screens available which is much quicker and safer. There are many other small changes that just add up so a better experience overall.
3. I still use my two SD cards for most of my music as they take no power, no data and are always available without having to hook the phone up (or playing via bluetooth). Also with two cards there's a massive capacity as well. On mine it's a CD / DVD player and I quite often find myself (when waiting for the wife at the train station when she goes on meetings), watching a DVD in car

4. Not used it myself (as most of my music is on SD cards) but might be something I'll try in future with either Deezer or Spotify?
5. I'm sure the new 6.5 screen will be leaps and bounds better than the older 5.8" screen as it now has the resolution of the Nav Pro.