Great idea.
Possible improvement: Only show the effect if the user uses the tab key. IMHO the effect is distracting if you use the mouse and are already looking at the focused object.
That's actually pretty similar to how in Linux you can choose to either have to click to give a window focus (like in Windows), or simply have whatever window currently is under the mouse to have focus.
I just released Flying Focus 1.2 that doesn’t do anything on mouse interactions.