Share your experience!
It's a good idea if you also forward your feedback/suggestions to your Local support. I'll give my opinion on some of your points:
1. There's a new interface presented in Z5, dunno if it will be extended to Z2 on Marshmallow.
2. If your phone is beginning to lag, consider performing a software repair using PC Companion after a back up of your data.
3. That's how Ultra Stamina mode is supposed to work, it will throttle performance and CPU usage to extend battery life.
4. There's a new way to control volume and audio in Marshmallow, what's your exact problem with volume control now?
5. Image enhancement will go off when using Ultra stamina mode.
6. You can unlock with bluetooth in Settings -> Security -> Smart unlock
7. Same as 2
8. What changes exactly? Give some suggestions. Personally I use Sony mainly because I like the UI the most.