Figured that you might appreciate a copy of the Reddit post over here:

It could do with some Celestial Colonnades, Cryptic Commands, Path to Exiles and a higher land count. Most control decks will play 25+ lands.

Further, Sphinx's Revelation is one of the main reasons to play U/W. If you are on a tight budget, Blue Sun's Zenith is an acceptable alternative.

Other cards to consider include splashing black for cards like Thoughtseize or Esper Charm, or playing the midrange game slightly with cards like Supreme Verdict.

Ultimately though, you will benefit from Wall of Omens, Verdict, Rev and the Colonnades most. Everything else is just secondary.

Budget alternatives to some of the options include: Faerie Conclave for the Colonnade (not great), and Sunlance or Condemn instead of Path.

I'd love to have both the Celestial Colonnades and the Cryptic Commands, but they're out of my price range, unfortunately. I like the idea of adding Path of Exile though. Where would I slot it? I think I could trim the Wall of Omens down to 2 or 3, and Kitchen Finks down to 3.

Also, I only have 3 blue permanents, which affects the Mirror-Sigil Sergeant quite heavily. What can I do about that?

Regarding splashing for other colors, eh, not a big fan.

+1

You could possibly play Spreading Seas as a means to screw with your opponent's mana (it seriously slows down Tron decks and most three color Modern decks) while also upping your permanent count.

You could also try Harbinger of the Tides - although the cost may be prohibitive. He's a decent tempo swing and he gives you some more interaction with the Spinter Twin decks (they tap their twinned Exarch, make a copy, trigger its ETB effect to untap the original Exarch and in response to that trigger you bounce the original Exarch).

I would suggest Faerie Conclave, but I don't think that you have a window to activate it before Mirror-Sigil Sergeant checks for a permanent. I believe that the Sergeant's ability won't even trigger at the beginning of upkeep if you control no permanents, so by the time you attempt to activate Conclave it's already too late.