The only good argument you gave is that Submerge is good against CB and that Harness isn't. That's completely true. But:
Why do you want to bounce a Dark Confidant if you can also have it? Dark Confidant is absolutely thé NUTS in this deck ^^.
Casting cost 1 isn't really dramatic is it? I know it's sorcery speed but common: casting cost 1 isn't bad.
Let me tell you about the upkeep cost: What do you want:
Pay 1 mana each turn and attack your opponent with a 4/5 goyf which he needs to handle or he loses
Pay 0 mana to bounce a Goyf, possibly removing it from the game. With no threath on the board.
Don't ask me the question, but what if he plays another Goyf when you have a Mind Harness'd Goyf? You would have died aswell when playing Submerge.
@ GUnit
I personally believe that my list is pretty good. Why can't I post my list to tell people about a possible new build of Tempo Threshold WITH (some) comments?
About the perfect sideboard:
There ís a perfect sideboard for your metagame. Some guys are just looking in a sort of tunnelvision. Just ask yourself: How does your metagame look like? What do I need to make these matchups better.
I think every sideboard should include this right now:
3-4 Spell Pierce
3 Pithing Needle
3 Pyroclasm / or some other card because you don't play red
3/4 Mind Harness / Submerge
And flex slots

