There is a TON of incorrect information in this thread. I don't really have time to go through and correct it all right now, but I'll address what I can.
First of all, Black absolutely does not improve the quality of your removal. It doesn't. Ghastly Demise is strictly inferior to StP in every way, and this comes from someone who has used both at many, many times. The fact that it cannot answer a Negator or even an opposing Confidant is extremely relevant, and can lose you matches. To say that this is made up for by running Diabolic Edict is a fallacy, and at most it gives you more removal spells at the cost of cantrips or threats. It's blatantly obvious that Red has the most powerful removal package, because it doesn't run a single spell that is dead in the combo matchup. Burn can always fly at your oponent's noggin. While it doesn't have spot removal as potent as StP or Demise, it more than makes up for it in reach.
Engineered Plague is a relevant point of contention, but remember that White has Tivadar's Crusade (or Hail Storm) and Red has PYROCLASM. Both are viable solutions to the Goblin Problem, and as Goblins is basically designed to deal with Plague now, I wouldn't count it as the strongest possible solution to the deck anymore.
Haunting Echoes is AWFUL in Thresh. Notice that Thresh runs zero cards that cannot be played for 4 mana or less (3 with Werebear). This is for a reason. The control matchups you refer to will probably be either Loam decks, Landstill, or Wombat. Vs. the first two, you need to have 5 mana up on your Main Phase, which is an issue in itself when both decks rape your manabase, and then resolve Echoes through disruption (in the case of Landstill). Vs. every one of those decks, Armageddon is a Turn faster, supports your game plan better, affects the present board state, and disrupts your opponent more effectively.
Deed + Nimble Mongoose = Not a combo.
As Dark Confidant is the most important part of the black splash (see the Herbig project -
discussed here), all the huge guys and flashy win conditions drive up the potential for Bob to screw you. Even with just Force in the deck I get nervous. Some of you are even advocating cutting cantrips with Bob in the deck! The whole point of fitting him into the thresh shell was to utilize the natural knowledge of your top library card to make Bob free draw. Cutting Portent, Visions, or Brainstorm when they all make Bob better is a mistake. You don't really need Predict or Mental note as much anymore, but both are potent with or without Confidant in play.
Duress absolutely is NOT better than Meddling Mage vs. Combo. I'm not going to get into the debate on whether or not he's needed in UGw thresh with you, because that will go on forever, but let's say you have to choose one or the other. Duress is a temporary solution that takes the most pertinent threat out of their hand. This works into your game plan how? What are you planning to do in order to capitalize on that disruption? Play a cantrip and a Mongoose? Meddling Mage is a
threat that they must answer to win the game, or slow down to play around. Either way, he buys you at least some time, where Duress may or may not. Note that this applies more to Non-TES combo decks, but is true there, as well, if you play tightly.
Portent vs. Mage - Mage disrupts your opponent starting turn 2. Portent draws you another counterspell that doesn't help vs. Storm. Note that I don't disagree that Portent is good, but I wouldn't choose it over Meddling Mage.
I may have more later, and feel free to disagree.