Why is this relevant? No one said anything about using Assault Vest Genesect to take hits from Heatran. This is why you use Assault Vest Genesect:
252 SpA Choice Specs Latios Draco Meteor vs. 0 HP / 0 SpD Genesect: 165-195 (58.3 - 68.9%) -- guaranteed 2HKO
-2 252 SpA Choice Specs Latios Draco Meteor vs. 0 HP / 0 SpD Genesect: 83-98 (29.3 - 34.6%) -- 7.1% chance to 3HKO
252 SpA Choice Specs Latios Draco Meteor vs. 0 HP / 0 SpD Assault Vest Genesect: 111-131 (39.2 - 46.2%) -- guaranteed 3HKO
Normally, Choice Specs Latios has a small shot at 2HKOing Genesect with two consecutive Draco Meteors, and Stealth Rock guarantees the KO. With the Assault Vest, Genesect can switch into Draco Meteor pretty easily and immediately put the opponent in a bad position with U-turn: they either lose their Latios or they switch out, something else takes some damage from U-turn, and they lose the momentum. That isn't the only example, either. Mega Alakazam can normally KO Genesect with Focus Blast followed by Shadow Ball, but with Assault Vest, Genesect is only 2HKOed by Focus Blast 27% of the time even after Stealth Rock (factoring in Focus Blast's accuracy, that is). Mega Gardevoir will normally almost always 2HKO Genesect with Hyper Voice with Stealth Rock down, but with Assault Vest, Genesect has a small chance to avoid even a 3HKO. Gengar needs a Life Orb just to have a measly 0.4% chance to 2HKO Assault Vest Genesect with Shadow Ball, whereas a Black Sludge variant would normally 2HKO 91.8% of the time if Genesect wasn't holding an Assault Vest. These are the sorts of Pokemon that Genesect pivots well against, and it does so because of its solid defensive typing, good Assault Vest-boosted special bulk, great coverage, and STAB U-turn. Even if it doesn't deserve a slash, don't knock it just because it can't take a Lava Plume from Heatran of all things.
1. CSpecs Latios is never used, and switching in on it is a bad idea, they are LO.
2. Switching Genesect into moves is a horrible idea, always.
3. You aren't touching a sub disable gengar with special attacks at +0
Focus sash genesect is a counter lead, and designed to score suprised KOs where Scarf/Banded can't. The fire moves that genesect worry about move are OHKO's even with AV, or are physical. You will see Sash Genesect in SPL for that very reason, being able to make clutch plays against stuff like lead Deo-s, revenging a Mega Pinsir that got out of control, even Mega Lucario at +2.
Genesect is not a pivot, its a revenge killer and a lead. Aegislash is a pivot, not genesect.