Sorry guys but Focus Punch seems super gimmicky to me (in fairness, you called it gimmicky yourself).
Focus Blast still hits Heatran harder and hits Tyranitar just as hard in a sandstorm. You'll put a dent in Blissey/Chansey but not nearly enough for a 1HKO.
If you hit Heatran/Tyranitar on the switch-in with Focus Blast, you'll do enough to force them to switch out or get 2HKOed. If you hit them with a Focus Punch on the other hand, you'll still be forced to switch out.
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Right now I'm convinced that Fire Blast/Solarbeam/Focus Blast/HP Ice should be the standard for ZardY, with Roost an option over HP Ice for some variants that have proper team support.
Well obviously you don't 1HKO the pink blobs but it's an almost certain 2HKO on Blissey if you follow up with Fire Blast and you can beat Chansey too with good prediction, have a look.
4 Atk Mega Charizard Y Focus Punch vs. 4 HP / 252 Def Blissey: 442-522 (67.7 - 80%) -- guaranteed 2HKO
252 SpA Mega Charizard Y Fire Blast vs. 4 HP / 252+ SpD Blissey in Sun: 184-217 (28.2 - 33.2%) -- guaranteed 4HKO
That looks like a guaranteed 2HKO after any hazard damage. You can win with good prediction even if you don't manage the 2HKO (see below).
4 Atk Mega Charizard Y Focus Punch vs. 4 HP / 252 Def Eviolite Chansey: 324-382 (50.4 - 59.5%) -- guaranteed 2HKO
252 SpA Mega Charizard Y Fire Blast vs. 4 HP / 252+ SpD Eviolite Chansey in Sun: 145-172 (22.5 - 26.7%) -- 32.9% chance to 4HKO
It's not getting past Chansey with a 2HKO but there is a bit of a prediction game here. Charizard can Focus Punch on anything except Seismic Toss for the 2HKO (Softboiled avoids the 2HKO but Focus Punch does more damage than Chansey can heal) and threatens to 2HKO with Fire Blast so it beats Chansey with good prediction since it can only 3HKO back with Seismic Toss.
4 Atk Mega Charizard Y Focus Punch vs. 252 HP / 0 Def Heatran: 214-252 (55.4 - 65.2%) -- guaranteed 2HKO
4 Atk Mega Charizard Y Focus Punch vs. 0 HP / 0 Def Heatran: 214-252 (66.2 - 78%) -- guaranteed 2HKO
It's not getting past Heatran, as you say. Does a nice chunk though so Heatran can't switch in again.
4 Atk Mega Charizard Y Focus Punch vs. 252 HP / 0 Def Tyranitar: 412-488 (101.9 - 120.7%) -- guaranteed OHKO
4 Atk Mega Charizard Y Earthquake vs. 252 HP / 0 Def Tyranitar: 138-164 (34.1 - 40.5%) -- 47.7% chance to 3HKO after Leftovers recovery
252 SpA Mega Charizard Y Focus Blast vs. 252 HP / 252+ SpD Assault Vest Tyranitar in Sand: 196-232 (48.5 - 57.4%) -- 93.4% chance to 2HKO (It's really less than 47% because of 70% accuracy twice)
This one needs no explanation lol.
As you say it is definitely a gimmick and HP Ice has way more utility but don't dismiss those calcs. :)