There is this nurfed_threat.zip mod, it was mentioned in a topic in the official us ui forum ... i never installed it - but from the code it should do something like "calculate" aggro for the raid based on different values for spells, skills etc
(not sure, if the page is german for you non german speaking guys: down the page choose "free", and on the next page wait (queue) until "Download-Ticket reserviert. Bitte 14 Sekunden warten." changes to "Download: Nurfed_threat.zip" ... "Nurfed_threat.zip" is then the clickable download link)
It's actually fairly accutate already - you just need to understand how the mod works. Sure, it's not perfect and could definantly use some work, but after doing some testing with our tanks, it seems to work pretty well.
A) You have to join the channel NurfedThreatSync. So do the tanks.
B) The mod is designed to be used against single pulls.
C) Tanks with Defiance have to have the defiance switch turned on, and priests with subtlety have to have that switched on (/nthreat defiance and /nthreat subtlety).
D) Your threat is only measured against that of the tanks, since that's who you need to stay under.
E) Mob deaggros do not work correctly. Death doesn't clear your aggro (which it should).