Ifrit's auto-attacks -should- elevate his DPS above that of Carbuncle or Garuda because they have similar damage on their abilities and the caster pets do not auto-attack. His main ability is also instant, whereas Carby and Garuda's has a small cast time. If you can't keep him alive, though, he'll contribute far less and he doesn't have Contagion so those are two big problems with using Ifrit.

You can probably ignore Ifrit all together and not really suffer, as the differences between the two types of pets should be pretty minor damage-wise.