MNK is one of the top, if not the top, single target DPS jobs right now.

The difference in DPS between all the jobs isn't that large overall, however they all do specialize in their own way of DPS. This is with the expectancy that a MNK will be forced to lose GL3 a few times during a fight due to mechanics. If there are no mechanics that lead to MNKs dropping GL3, then they are ahead by a significant margin in DPS.