A lot of dps "feeling gimped by certain tanks" comes down to specific encounters, specific tanking assignments, usage of misdirect/tricks of the trade.
So I guess the questions are:
1) On what encounters do the threat problems show up?
2) AoE or single target threat?
3) Are your Hunters/Rogues misdirecting correctly? They may think one tank has better initial threat than another because the other tank habitually gets Misdirect, while the "bad" tank generally doesn't.
4) What classes are pulling aggro?
Kathy, I said, "I'm lost" though I knew she was sleeping
I'm empty and aching and I don't know why
Counting the cars on the New Jersey Turnpike
They've all gone to look for America