All the 15 mana teleport change did was make sure the sovereign never cast any spell but teleport. He's essentially a teleporter now.
This is the problem; teleport in itself is more powerful than all the other sovereign spellcasting capabilities combined. It was completely ludicrous before when you could teleport and cast spells with the same champion with total impunity. Heck, you could imbue a champion and a few level-ups later he could be teleporting too.
Now teleport is reserved for powerful channelers, not the run of the mill kind you've recent imbued, and it's going to deplete your spellcasting reserves. This is a good thing, forcing you to split up your forces rather than running around with one big stack of death filled with superpowered summons and crazy-strong casters.
I think right now there's a severe problem that people have grown to use teleport as a crutch to deal with monster spawns and defending their empire. It's hiding the real issue that the only half-decent military tech in the early game is archery (and even then you need squads to make them effective) and that the only decent melee units are summons. Even champions and sovereigns are complete jokes in melee until you unlock the higher-end equipment that actually lets them survive, and even then one bad roll and they're dead.
Personally, I think teleport needs to be nerfed further. I like the idea of splitting it up into multiple versions with different strengths and limitations, but I still think all of them should require research to be unlocked. What needs to change is that basic unimbued champions need to be more powerful (especially in melee) and that half-decent military equipment needs to be unlocked earlier so we aren't dependent upon magic to defend us until we have the 8 or 9 research levels in the warfare tree necessary to actually build "real" military units.