Once you have a Mark 14 Epic Ground weapon, then it is at its max. The only way to improve its DPS is through the Weapon Proficiency skill and set bonuses like the ones from the Terran Reputation (Disruptor), Dyson (Proton), Honor Guard/Adapted MACO (all damage), and Temporal Reputation (Antiproton). The other possibility is increasing your Critical Chance and Critical Severity. There are other set bonuses that increase DPS, but the damage increase is based on probability and not guaranteed.