Not really worth it on either of them
252 Atk Beast Boost Kartana Black Hole Eclipse (120 BP) vs. 0 HP / 4 Def Charizard: 107-126 (69.9 - 82.3%) -- guaranteed 2HKO
252 Atk Beast Boost Kartana Corkscrew Crash (140 BP) vs. 0 HP / 4 Def Charizard: 93-109 (60.7 - 71.2%) -- guaranteed 2HKO
252 Atk Beast Boost Kartana All-Out Pummeling (175 BP) vs. 0 HP / 4 Def Charizard: 77-91 (50.3 - 59.4%) -- guaranteed 2HKO
252 Atk Beast Boost Kartana Breakneck Blitz (200 BP) vs. 0 HP / 4 Def Charizard: 177-209 (115.6 - 136.6%) -- guaranteed OHKO
With a SD on the switch, even Knock Off can get the KO. It MIGHT help in a 1v1 scenario, but I don't buy it. What's more, you can get a functional equivalent of the move without loosing the chance to use a Z-move somewhere else by using a Belue or Charty Berry and Natural Gift. You even get a technically better chance to break through Heatran and Salamence due to how damage is rounded. Assuming Rock type:
+2 252 Atk Beast Boost Kartana Natural Gift vs. 212 HP / 0 Def Heatran: 139-164 (72 - 84.9%) -- guaranteed 2HKO after Stealth Rock and Leftovers recovery
+2 252 Atk Beast Boost Kartana Breakneck Blitz (200 BP) vs. 212 HP / 0 Def Heatran: 138-163 (71.5 - 84.4%) -- guaranteed 2HKO after Stealth Rock and Leftovers recovery
252 Atk Beast Boost Kartana Natural Gift vs. 4 HP / 0 Def Salamence-Mega: 118-140 (69 - 81.8%) -- 50% chance to OHKO after Stealth Rock
252 Atk Beast Boost Kartana Breakneck Blitz (200 BP) vs. 4 HP / 0 Def Salamence-Mega: 117-138 (68.4 - 80.7%) -- 43.8% chance to OHKO after Stealth Rock
You can even swap the nature to Naive (you will die to specials attacks regardless) and use a Wiki Berry instead for extra utility outside having that one-time anti-Zard nuke, mostly in being able to take on Metagross and Tapu Fini more consistently, altough it will notably deal less damage to Salamence.
252 Atk Beast Boost Kartana Natural Gift vs. 4 HP / 0 Def Salamence-Mega: 94-112 (54.9 - 65.4%) -- guaranteed 2HKO after Stealth Rock
As for Volcarona:
252 SpA Flame Body Volcarona Inferno Overdrive (185 BP) vs. 0 HP / 0 SpD Charizard in Sun: 139-164 (90.8 - 107.1%) -- 43.8% chance to OHKO
252 SpA Flame Body Volcarona Breakneck Blitz (200 BP) vs. 0 HP / 0 SpD Charizard: 134-158 (87.5 - 103.2%) -- 25% chance to OHKO
Not even worth discussing. Fire Blast does more damage thanks to Zard's sun. Even if you don't want a Firium-Z, there are coverage options that also kill after a single Satk boost and have other situational uses:
+1 252 SpA Flame Body Volcarona Shattered Psyche (175 BP) vs. 0 HP / 0 SpD Charizard: 175-207 (114.3 - 135.2%) -- guaranteed OHKO (toxapex+rotom-H)
+1 252 SpA Flame Body Volcarona Supersonic Skystrike (185 BP) vs. 0 HP / 0 SpD Charizard: 186-219 (121.5 - 143.1%) -- guaranteed OHKO (anti-rain)
If you are good enough and face incompetent enough opponents, you can win with an Unown mate. And I am willing to bet Vivillion, Gengar and Mimikyu did most of the job laddering up. And I also bet that you only used Focus Blast in 1/15 matches (only counting ones that Gengar actively contributed in). Inacurate moves can and will betray the user sooner or later, they can help on the start but you will never be able to stay on the top with them for hopefully obvious reasons.
I've never wanted to dwelve nor care about past gens. I only mentioned them at all because you commented Snorlax used to be bad, and he never was.
No decent player will EVER spam Magma Storm against Moltres, they will just keep switching in and out, taunting him if necessary, taking advantage of his pathetic moveset that doesn't even have Toxic to prevent dragons, keldeo, tapu fini, suicune, or Marowak from coming in for free. I've seen that (CENSORED) stall team dozens of times, and I can tell you, they never survive more than 50 turns before eating a 6-0.
Kartana however does not learn Natural Gift and Charizard is just one of many matchups in which Kartana can use z-breakneck blitz. It's the same with volcarona, you can do the charizard calcs (though you got them all wrong, these are regular zards, not zard x for example which takes nothing from z-fire. As for some of what you are saying about volc, other than flame body, mega swampert outspeeds and always ohkos volc.
You've literally done calcs where you've made up the moves a pokemon learns. I doubt someone who doesn't even know what moves Kartana knows has much knowledge of the top of the tier.
As for OU, it's kinda irrelevant but obviously you dont use magma storm on moltres, you bait it out and switch moltres in on it, allowing other mons like mega sableye to stall out heatran and prevent stealth rock or prevent it from trapping toxapex.