Not to mention, improved HP update times have been implemented in both the default UI and any mod wanting to use them since Lich King went live. Without using any extra libraries.
A quote by Haste in the comments here: "I don't really see why you would write a plug-in for this. The predict system which was added to the game client during the wotlk beta does a better job at predicting the actual HP than Quick Health and Instant Health. The Quick Health library also doesn't have a maintainer afaik."