I'm surprised nobody has posted this yet or referenced to it but I ran some basic Treasure Hunter potency testing after logging in for the first time tonight and I LIKE what i see. I posted the info in the TH >>> Melee thread but I think it's big enough news it deserves its own thread. The test data is located here.
Summarizing my test results: Treasure Hunter is applied the moment we make our first action upon a mob and includes all TH+ item bonuses then and there. This effect will not diminish if we unequip those items, and so long as the mob does not go yellow the highest level of TH stays upon the mob straight to the kill. In other words the thief's knife can be treated like a regular piece of TH + armor and only needs to be equipped at the very start of the fight. Once we perform our first action upon a mob equipped with Thief's Knife + whatever armor we have we can unequip that armor and swap to a useful offhand with no consequence to the TH rate. Oh by the way, the thief's knife does work in the offhand for the opening strike.
Now I would still fulltime the thief's knife when farming dynamis or salvage but that's only because it involves fast kills on scrub mobs with highly desirable drops (salvage cells count as highly desirable). There are very few examples of scrub mobs that die in seconds outside of those two events with drops that are worth using TK for (I can only think of water farming for Olla Grande offhand). Currently there is the example of Magian Trial upgrade items which can also be bazaared, but that will change as supply outweighs demand over time. I think this is good news because the place where Treasure Hunter matters most is NM's, and I can live with holding my TK for my opening attack if it means I won't need it for the rest of that NM battle.
EDIT: I just did some extra testing referenced HERE. The summary of that testing is that treasure hunter equipment can be applied at any point in the battle. You can start the battle off in DD equipment and swap to the TK + TH + armor at the very end or begin the fight with it and it makes no difference. So long as you have all of your TH + items equipped at any point during the battle you only need them on for ONE action and they can be removed with no consequence thereafter.
Edited, Dec 16th 2010 12:52am by Melphina