Taglist does something similar, and it uses ':h updatetime' to toggle how often it works.
My thoughts exactly, that would be awesome!
That's a lot of "git diff" calls!
Or a reimplementation of the algorithm in VimL ;)
In the Sublime version, one of the things I did to avoid that is it doesn't call `git diff` if it has been less then 5 seconds since the last `git diff`.