I would say both DRG and the MCH/BRD would need buffs if Disembowel were removed. It's 5% of the Dragoon's damage too. Taking disembowel away without buffing it at all puts it not only lose between 300 to 600 rDPS, but also about 276 personal DPS based on this average.

Could also use results like this to show it off. With piercing counted, DRG is still weaker in total than the Monk. Now subtract 300 from DRG, and he falls to 6698. He's now leagues below the other melees. Now multiply that by 0.95 because of his own lost disembowel, and that drops even further to 6363 total dps.

Remember the MNK is at 7,248 and the NIN remains at 6,924. The Bard's total dps doesn't change, staying at 6,707 because FFLogs filters the piercing out in that total dps either way. The Dragoon sans piercing is the weakest DPS in this speedkill. By about ~340 dps.

I do not support the idea that DRG should be buffed in this theoretical no-piercing world but MCH/BRD shouldn't. However, DRG suffers worse for losing piercing than MCH/BRD would.