Flaming Arrow hits Faust, too, so it is a DPS gain for you. If anything, it just inflates your numbers as the adds may get caught in it too. Whoever told you that Flaming Arrow is a DPS doesn't know what they're talking about. It is worth a total of 350 potency (35 potency x 10 ticks), and if Faust stays in the AoE for at least eight ticks it is a DPS increase over Sidewinder (250 potency, but in one shot).