A very gifted guildie of mine (
Meloeth, EU-The Maelstrom) wrote an addon that uses combat log events and information to calculate health amounts as they occur rather than waiting for the UNIT_HEALTH tag from the server. It's been extensively tested in our guild the last few months and he's finally released it:
Instant Health | World of Warcraft Addons | World of Warcraft @ Curse.com
Healing using this and grid/pitbull was error-free and the difference immediately noticable. Health bars are extremely reactive to all incoming/outgoing changes. You will also notice that your and other's health bars dip into the negative before the server catches up and actually 'kills' you. It is possible, though rare, to save someone who dips into the negative if they are healed before the server catches up, or also to avoid a hit that would otherwise kill you by vanishing/blocking/bubbling.
The single detriment is that Recount and other damage meters will not show accurate healing meters using this addon. As the health update is instant, the meter waits for the unit_health message and then tends to view most healing as overhealing.**
**edit: Meloeth has updated his code and now all damage meter and combat text addons should play nicely. Make sure you download the newest version on the Curse Page!!!
In any case it is a large improvement and I recommend it to everyone. It is also something I haven't seen before which is why I thought it deserved it's own thread.
Meloeth has explained the mechanics further on his Curse page, Enjoy!