So for what to drop, if it were me I'd likely do the following:
-2 Dryad Militant
-1 Imposing Sovereign
-2 Banisher Priest
-1 Aurelia's Fury
+2 Chained to the Rocks
+1 Spear of Heliod
+3 Brimaz, King of Oreskos
My rationale is as follows:
Dryad is easily the weakest of your one-drops. Boros Elite will very frequently be a 3/3 going into combat which means it's already outperforming Dryad. Similarly, Soldier of the Pantheon can gain life, deals with BTE, and can even stay put to block bigger stuff like Obzedat. As such, losing two Dryads will sill leave you with 10 one-drops, but you've cut a few of the weakest link.
Imposing Sovereign is a solid card when you're just trying to outrun the opponent; but in reality it's easy to kill, becomes a quick target, and you'd likely rather be dropping either two one-drops, a one-drop and Chained, Precinct Captain or Daring Skyjek on T2. So, as such, going down to 2x Sovereign will still let you draw it on a regular basis, but won't always stick you with one when you don't want it.
Banisher Priest is a double-edged sword - he can be your best friend OR it can really just underperform and you've dropped three mana into a creature that did nothing. You're going up to 4x Chained, so really, you're not losing any removal power here by dropping down to 2x.
Adding another Spear will help ensure you get it. It might seem like it's not adding that much firepower, but T1-Soldier, T2-Precinct Captain, T3-Spear is one hell of a start and the pump works with the tokens created by both Captain and Brimaz, making an army of 1/1s (normally not a big deal) into an army of 2/2s (which has been known to kill people quite often - see Mutavault if you think a 2/2 isn't valuable). Sure, drawing a second one sucks, but then you have a backup as people will start packing more enchantment hate as Theros and BNG become the core cards in standard.
Brimaz, well, he's the cat king. By having a 4th toughness he survives Bile Blight, Lightning Strike, Anger of the Gods, and, well, a LOT of the removal out there. That in mind, he's a major threat that survives a lot and just oozes value.
You likely want to keep Boros Charm for the utility it provides against sweepers and even the straight burn it can offer to finish off an opponent with a low life total.
As far as Gideon and Angel are concerned, well, they are VERY useful SB cards. Against R/G builds - which are a clear tier 1 deck now thanks to Xenagos, God of Revels. You drop Gideon on T4 and he's an immediate distraction, forcing them to attack him and not you - often times buying enough turns for you to stabilize or otherwise assemble a winning alpha strike. His +1 ability not only makes it take forever for them to kill him, but it also makes him an ever-increasing threat. He could side in against R/G (as they like to flood the board) OR control to quite a bit of success. Angel fits much of the same bill in terms of her impact. Yes, she dies to quick removal just like everything else, but if they've been sitting there trying to take out all your little guys to keep ahead of the curve, well, she comes in and they're likely out of removal by that point. Alternately, she could let you get 3 guys from your yard back to your hand... "Why yes I'll happily get those two copies of Brimaz and a Banisher Priest back." Toss in the fact that outside of Stormbreath Dragon she literally outclasses ANY threat on the board and, well, that's the making of a solid SB card. Obviously you leave her out against decks like MBD, but she's solid against R/G, mono-blue, things of that nature. AND, even better, with Nykthos in the deck you'd be surprised how easy it is to get to 7 white care of things like Chained, Precinct Captain, Spear, and Brimaz.
Again, they're likely SB cards, but they let you turn what is a pretty vanilla aggro deck that would get hosed by well timed sweepers into something that is able to last more into the mid-range. Also, you can't bank everything on Assemble saving you as cards like Golgari Charm, Drown in Sorrow, and Bile Blight can easily clear away your token army for them to swing in for fatal. Personally, Elspeth is a better option than Assemble anyway, as she actually will outpace it for the first six or seven turns (by which point she's already used her ultimate and won you the game).
Feel free to take or leave the thoughts but I'm tinkering with W/u (splashing for Ephara and D-sphere) with Gideon, Elspeth, and Angel in the main and it's actually performing quite well.