I played the Uw control list for a league and felt like the cards were too specialized. They could be good but unnecessary or just irrelevant. If you splash white terminus is a good sb card and in all builds counterbalance is an option (considered it before). I didn't post remarks before on the list because I had a very limited number of games with it.
Mentor is probably not worth splashing, I think Bitterblossom was better.

About the changes to my list I decided I will just forget the idea of beating grixis control reliably. A bit like vs chalice decks you do still win some post sb games without any particular strategy accomplishing and vs control game ones are a lot easier, so I think it is more efficient to use the slots to focus on combo decks (or others if needed, like massacre) and accept to have grixis as a difficult match-up. Which has been a good choice given the range of decks I faced since I decided myself about this change. Besides some of the slots may be useful vs hymn or surgical (spell pierce which has the good idea to hit jace, cb and sneak attack, and surgical that can mess up with scm or "counter" a surgical).
Spell pierce #2 could be something else (cb, LDV to bring in vs chalice decks, ...) and possibly 3rd surgical too. With 3 I am happy to side in 2 vs a lot of decks, but 3 vs the explosive graveyard decks.
I could add more cards vs thalia decks but I don't face them so often and massacre isn't great vs 5c humans. It's possible Hideous laughter comes in before.
Since I have more cards vs combo I replaced Dimir Charm with Echoing Decay even if the charm was good.
Playing with Thought Erasure instead of 4th seize since intuitively it seemed nice to lose less life and you sometimes have fow and useless discards with no time to delay the combo. The surveil is useful. Could become 2 or 0, but 1 is good so far. I have a 9th fetch which has downsides but is probably safer. I don't have a problem sideboarding out a swamp now vs control or combo because I have less black sb cards and the 9th fetch.
Among the things I tested Mission Briefing was a bit too expensive (as a wish slot but also too often when I sided it in). I briefly tried again tainted pact, it's still on my mind. One should test with it or Spoils of the Vault in mind as a wish target.
Murderous Cut has situations where it's inferior to Fatal Push but it happens less than the other way around I think.
Didn't do great vs Turbo Depths and an infect deck, I am thinking about snapback a lot again.

I'd like to find a sb card that acts as a combo piece. Decree of Pain is a good card for this matter but it will be useful vs Thalia decks and I would like the sb card to be sided in mostly for when I sb out LDV and/or Intuition. Stormtide Leviathan was a consideration in 2013 but is too fragile vs reb or Flickerwisp (and people still have stp post sb quite often). It could be the third emrakul or a griselbrand, but I would like to find something better by itself and/or solving difficult situations.

4 Omniscience
4 Cunning Wish
4 Polluted Delta
3 Scalding Tarn
6 Island
4 Brainstorm
4 Ponder
4 Preordain
3 Thoughtseize
3 Force of Will
4 Show and Tell
2 Underground Sea
2 Lim-Dûl's Vault
2 Emrakul, the Aeons Torn
1 Intuition
3 Baleful Strix
2 Bloodstained Mire
2 Swamp
2 Inquisition of Kozilek
1 Thought Erasure

1 Noxious Revival
1 Firemind's Foresight
1 Eladamri's Call
3 Surgical Extraction
1 Release the Ants
1 Murderous Cut
1 Cryptic Command
1 Force of Will
1 Baleful Strix
1 Skeletal Scrying
1 Echoing Decay
2 Spell Pierce