It depends on which gunman is present. The best ones to get are the sombrero gunman, and the leader himself. Still, the weaker gunmen can KO earlier than before, which includes even the weakest gunman of the pack.
Normally the weakest gunman can't KO Zero Suit Samus until her current damage goes over sudden death levels, but my buffs enable him to KO Mario when he goes over 275% damage. And this also means that he can KO the lighter fighters sooner.
Also, the custom variants of Wild Gunman (Quick Draw Aces and Mega Gunman) have been buffed too.
Mega Gunman gets the biggest buff, where the giant gunmen can deal more damage and knockback; they are very slow to attack, but when they do attack, their shots will pack a punch with their knockback. In fact, Mega Gunman is buffed up in that it's now the strongest in damage AND knockback; the knockback is almost comparable to Little Mac's KO Uppercut, but the shots can still be blocked by shields. Even the weakest gunman can be quite nasty with his shots, but as usual, he makes KOs later than the other gunmen.
Quick Draw Aces deals more damage than Wild Gunman, but the shots have less knockback to compensate for how fast the gunmen can attack. Second strongest in damage, but the weakest in knockback. In fact, the Quick Draw Aces version of the weakest gunman can't KO Mario until his current damage goes over 330%.
On a side note, the default Wild Gunman deals the least amount of damage, but is the most balanced with the knockback (stronger than Quick Draw Aces, but weaker than Mega Gunman).
As for KO percentages against Mario, I did set up a table after doing a good amount of tests with the hack. Keep note that the KO percentages can be lower for the lighter fighters, and higher for the heavier fighters.