T10 SRs aren't really the best places to judge DPS since they're basically built on cheesing AOE.

That said, NIN > DRG/MNK.

NIN contributes more overall party damage and does roughly the same DPS as MNK, if not slightly higher. All without having positionals. Weeee.

DRG also does fantastic DPS, but is behind MNK - it makes up for this by increasing the BRDs DPS. The problem with this is that the BRD needs to be exceptional or at least good to warrant DRG over MNK in terms of DPS.

MNK buffs Ifrit's DPS and provides an all-important INT down which is pretty much a god-send for progression (also the only useful Mantra). The INT down allows for the healer to potentially DPS more, depending on how the fight works and all that stuff. MNK's major downside is that it falls flat in fights where it consistently loses GL3, whereas DRG/NIN don't have that problem.

That saaaaaaaid the only real point besides speedruns to have min/max DPS is for progression. You will, 9 times out of 10, want a MNK for DK and Mantra over NIN/DRG for their pure DPS output. You can basically play any of the 3 and be fine, but MNK receives a huge boon in progression content due to their utility.