Just found out the deck is capable of beating control builds with more consistency. I mean, I don't immediately lose if I have a match against a control deck, that will only happen if I make too many mistakes. The flow of threats posed by this deck is often sufficient to out-card the hand of the opposing control player :)
Against Monoblack, it has also been behaving well. I'll write down what I think should be a good sideboards for each archetype.
Monoblack
The ideal scenario is to side in 2 Pithing Needle for the Pack Rats, 3 Arbor Colossus against Desecration Demon and Nightveil Specter and one extra Sylvan Primordial, against Whip of Erebos and Underworld Connections. We may side out 2x Courser of Kruphix, 1 Karametra, 2 Voice of Resurgence and one Archangel of Thune, as we get a more round-up deck to counteract monoblack's main perks.
Monoblue
Our white removal, in the form of Banisher Priest and Angel of Serenity, does a good job taking care of Nightveil Specter, Thassa, God of the Sea (when turned into creature), Master of Waves and Tidebinder Mage. But we can always make their lives more difficult by siding in 4 Mistcutter Hydra (against their whole plan) and 3 Arbor Colossus (against Nightveil Specter and Cloudfin Raptor. We can side out 4 Voice of Resurgence, as our opponents don't do much on our turn, 2 Courser of Kruphix, as their only role will be basically avoiding bad draws (since it will be tapped for most of the game) and 1 Armada Wurm.
Azorius/Esper Control
Now the deck can face them up with courage. Side out everything that doesn't have a role here: 4x Banisher Priest, 3x Trostani, Selesnya's Voice, 2x Karametra, God of Harvests *f-etch*, 1x Armada Wurm and 1x Archangel of Thune. Side in 2x Spirit of the Labyrinth, 2x Pithing Needle, 4x Mistcutter Hydra and 3x Savage Summoning. The deck is now much more aggressive and the Spirits really do their job, as I expected.
Aggro builds (White Weenie/Selesnya Aggro/RDW/Rakdos)
Really, this deck is just a monster against Aggro builds. You don't really need to change much, as most cards here have high toughness (thus survive easily to damage removal) and Trostani, Selesnya's Voice just RULES. I always laugh when I get a turn 2 Caryatid, turn 3 Trostani and a turn 4 turned-on Karametra. The damage you may have taken in the beginning of the game is just history in no time. The most difficult build to play against is Rakdos, because they have access to direct spot removal, but nevertheless the flow of menaces on your side usually suffices. Against builds using red - hence having access to Chandra's Phoenix and Stormbreath Dragon, just siding in 3x Arbor Colossus (siding out 2x Spirit of the Labyrinth and 1x Sylvan Primordial) usually makes the cut.
Green/RG Monsters
Again, this deck mainboard usually outruns these builds. Trostani, Selesnya's Voice does a monster job here too, and if you happen to pair her with Archangel of Thune, your creatures will always be larger then their creatures. Should they kill your Archangel with an Arbor Colossus, just bring her back with Angel of Serenity. A kind of sideboard similar to the one we make regarding aggro decks can be made here - siding in 3 Arbor Colossus and 1 extra Sylvan Primordial in place of 2 Spirit of the Labyrinth and 2 Voice of Resurgence. Really, what I usually see is the opponent trying to come up with solutions to my threats, and not the contrary, which is always a great advantage.
I think that's about it. I've been working on this deck for some time and it always seems SO FUN to play with. Although it's made only by creatures (and Garruk), they all do something else besides standing there and attacking, and the order you do things DO matter. I'm bringing my creatures to the board all the time, but I'm also always thinking on how best to counteract my opponent's strategies, be it by applying more pressure, building my own board to out-race my opponent or just waiting for the best time or designing the best order of casting cards to out-wit my opponent. I think it really gives me a good Magic experience overall. :)