Through the powers of mystical rainman mathing, Ifrit on Sic will generally do about 15~ or more DPS than Ifrit on Obey even if he only hits 1 target with his AoE ability. This will vary based on your gear obviously.

So you gain DPS by letting him do as he pleases, though you'll want to put him on Obey regardless if you're fighting a mob where Stunning is needed as Ifrit will just spam it and then the enemy will build resistance and such. Putting him on Obey for a quick moment to time your Radiant Shield usage isn't going to hurt too much either.

But Sic for ifrit is not only fine, but better for DPS. Just use it wisely.