The problem of just giving us (and the same logic applies to SAM too) enough personal damage to trump other classes personal damage plus offensive utility (let's call it tDPS short for total DPS) is not dungeons because those are already unbalanced by default and are quite frankly super easy with any standard group, the problem happens in low skill raids, right now the strongest class in the game in terms of tDPS is probably NIN, however a super skilled NIN in a low skill party (read a party that can do the mechanics but by playing ridiculously safe and at a giant dps cost) probably has way less impact than a skilled SAM due to the rest of the party profitting much less from TA, this is why selfish dps is probably impossible to balance as the level of dps you'll have to give to them to be viable in high end groups, will mean they can basically carry the dps of bad groups.

Magic vulnerabity is IMO a much better idea than just potency increase because our tDPS will somewhat depend on our group capacity to capitalize on the debuff.