Android kitkat felt a lot snappier at the first installation, but after I had reinstalled xposed, snappiness decreased a lot. So if you are using the Xposed framework, consider going Vanilla again.
livingwogoogle also can speed up your phone as Google Apps tend to use a lot of RAM and CPU.
There are non.
No.
PS.: Trying to hold on to my FP1 until at least January 2018. 