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).