I am going to swim against the thread stream here, and say let SC in, and do not worry about the power level. If you were playing true Core only, that might be a problem, but given the power level already found in Forgotten Realms material, it will just not matter.
That said, I agree with Steel Wind that you should consider using some SC spells for your NPCs as well. That way, if a particular spell is more powerful than the norm, it will be self-balancing as the players have to deal with it just as the GM does. (Sound Lance, I am looking at you, you lovely overpowered thing.)