Some quick testing since I think that "Data Collection" is broken? (I would really like to know what Ifrit's DPS is now with the same i130 Gear)
(Dreadwyrm Grimoir)
Bio Damage
336 DET + 408 SPD + 658 INT = 152-168
268 DET + 590 SPD + 663 INT = 157-174
305 DET + 441 SPD + 663 INT = 153-170
So it looks like Spell Speed does impact dots quite a bit.