Ouch. No no no no. You did not understand the whole mana concept.
+12% mana means 1 tile is worth 1.12 mana instead of 1.0 . You don’t get any mana for free… (only in Ninja tower as one of the Oni-boni)
You still need to charge the hero with tiles.
Khagan (slow) needs 12.0 to be charged. Say you played 10 tiles (none ghosting) with the bonus then you have 11.2 mana. Not quite enough. So you need the 11th tile to charge him.
Khagan 12 -> 11
It is interesting bringing average heroes (10 tiles) together with Khagan.
Now 9x1.12 = 10.8 …so 9 tiles are enough to load that average mana hero.
Average 10 -> 9
This is what you wanna do: Bringing Average heroes together with Khagan to make them faster.