Well HT and ID does give bonus potency when hit on the right spot (100 to 150 HT and 100 to 180 ID) so I don't think "position reqs." are the cause of the Job's subpar DPS. I personally as a DRG player love the gimmick of landing HT and ID on their correct positions - it's our thing! What we lack is an inherent ability to self-buff ourselves.

MNK has GL which gives 9% ATK and 5% SPD per stack, which totals to 27% ATK and 15% SPD at GL3. Ninjas have Futon (15% SPD). Thus both Jobs have the ability to dish out more DPS due to their low GCDs. DRG does not have its own version of those inherent buffs.

NIN has Raiton every 20 secs which deals 360 potency while DRG has Leg Sweep every 20 secs which deals 120 potency, 1/3 of what Raiton hits for.