Don't compare it to a set that hasn't been released yet, compare it to the currently available items.
Shiny colt only can make little more dmg than David or Lonesome, but if we looks David set bonus better and Lonesome had nice long run, only maybe little short. At duels still some more skill points better than little higher dmg. Others not released what you said, so I can only compare Shiny colt.
David firearm after upgrading it to lvl 2, have got same dmg as Shiny colt, if I remember correctly and David has got set bonus too, what more important at duels.
Lonesome have got again same dmg bonus if upgraded to lvl 2 and set bonus still a thing.
If we compare not upraded ones, Shiny colt has got about 35 more dmg, but no set bonus atm, because we don't know what will be it. More hit chance (or better dodging) or we go for highest dmg. Sometimes safer making little less dmg (opponents not always have chance have dmg buff) so if we using polishing stone we can't talk about less dmg after that. Possible use dmg buff with Shiny colt too, but that gun bonuses so less than others too, so if we making one less hit or getting one more hit, all plus dmg can't make more difference. Because if opponent good as you like, maybe you can hit 3-4 times, and what dmg bonus we get with Shiny colt and making, hit one time less or get one more hit sometimes can be deadly. You can ask other duellers guy, they better like using set guns for more skill points. Atm duels skills lot of time better than little more dmg at duels.