with only 1 bad roll is 15 (3/6/6), putting it only slighly higher.
Slightly higher? O_o
Do half more damage than average ... and only 3less than perfect roll is "slightly higher" ? O_o
And if your Sorcerer would have Charisma 20 ... wich is easily achievable and in long run desired i would dare to say for most builds ...
You would only give 15 or 18 damage with spell that have 10.5 average ... meaning either perfect roll, or only 3 (1/6 of that perfect roll) points weaker ...
I dunno, still seems pretty powerfull upgrade. :-/