So I've noticed this in Yanmega's OO:
Psychic hits Nidoqueen, Crobat, and Tentacruel harder than Air Slash but provides no other coverage, making it another very situational move
I don't know about you, but Psychic seems waaaay more appealing than Air Slash on the Speed Boost set: it still covers Fighting-types but also nails Poison-types which would require far less whittling for Yanmega to break through, and Air Slash doesn't seem to hit that much that Yanmega can't hit with Bug Buzz and Ancientpower.
252+ SpA Life Orb Yanmega Air Slash vs. 248 HP / 0 SpD Crobat: 196-231 (52.5 - 61.9%) -- 98.8% chance to 2HKO after Black Sludge recovery
252+ SpA Life Orb Yanmega Psychic vs. 248 HP / 0 SpD Crobat: 312-369 (
83.6 - 98.9%) -- guaranteed 2HKO after Black Sludge recovery,
guaranteed OHKO after Stealth Rock
252+ SpA Life Orb Yanmega Air Slash vs. 180 HP / 0 SpD Nidoqueen: 187-220 (51 - 60.1%) -- guaranteed 2HKO
252+ SpA Life Orb Yanmega Psychic vs. 180 HP / 0 SpD Nidoqueen: 296-351 (
80.8 - 95.9%) -- guaranteed 2HKO
252+ SpA Life Orb Yanmega Air Slash vs. 0 HP / 0 SpD Nidoking: 207-243 (68.3 - 80.1%) -- guaranteed 2HKO
252+ SpA Life Orb Yanmega Psychic vs. 0 HP / 0 SpD Nidoking: 328-387
(108.2 - 127.7%) -- guaranteed OHKO
252+ SpA Life Orb Yanmega Air Slash vs. 4 HP / 0 SpD Tentacruel: 140-165 (46.3 - 54.6%) -- 9.4% chance to 2HKO after Black Sludge recovery
252+ SpA Life Orb Yanmega Psychic vs. 4 HP / 0 SpD Tentacruel: 221-263 (
73.1 - 87%) -- guaranteed 2HKO after Black Sludge recovery
252+ SpA Life Orb Yanmega Air Slash vs. 240 HP / 0 SpD Tentacruel: 140-165 (38.7 - 45.7%) -- guaranteed 3HKO after Black Sludge recovery
252+ SpA Life Orb Yanmega Psychic vs. 240 HP / 0 SpD Tentacruel: 221-263 (
61.2 - 72.8%) -- guaranteed 2HKO after Black Sludge recovery
252+ SpA Life Orb Yanmega Psychic vs. 0 HP / 0 SpD Infernape: 343-406 (
117 - 138.5%) -- guaranteed OHKO
252+ SpA Life Orb Yanmega Psychic vs. 0 HP / 0 SpD Conkeldurr: 367-434 (
104.5 - 123.6%) -- guaranteed OHKO
252+ SpA Life Orb Yanmega Air Slash vs. 0 HP / 252 SpD Assault Vest Conkeldurr: 221-265 (62.9 - 75.4%) -- guaranteed 2HKO
252+ SpA Life Orb Yanmega Psychic vs. 0 HP / 252 SpD Assault Vest Conkeldurr: 179-213
(50.9 - 60.6%) -- guaranteed 2HKO
252+ SpA Life Orb Yanmega Psychic vs. 172 HP / 0 SpD Assault Vest Machamp: 200-237
(54.9 - 65.1%) -- guaranteed 2HKO
252+ SpA Life Orb Yanmega Psychic vs. 0 HP / 0 SpD Machamp: 296-351 (
92.2 - 109.3%) -- 56.3% chance to OHKO, 93.8% chance to OHKO after Stealth Rock
252+ SpA Life Orb Yanmega Psychic vs. 0 HP / 0 SpD Heracross: 270-320
(89.7 - 106.3%) -- guaranteed OHKO after Stealth Rock
252+ SpA Life Orb Yanmega Psychic vs. 0 HP / 4 SpD Roserade: 250-294
(96.5 - 113.5%) -- guaranteed OHKO after Stealth Rock
All in all, I don't see the point of Air Slash on Speed Boost Yanmega. Sure it has that nice flinch chance, but Yanmega should probably be prioritizing OHKOing stuff rather than hoping for a flinch to not die in return. Psychic just seems way more worthwhile, even for just the sole purpose of smacking Crobat much, much earlier.
Edit:
Air Slash does significantly more than Psychic to Fighting types (Air Slash KOes Bulk Up Conk and does a big chunk more to AV, does more to Cobalion), more to Gyarados and Tornadus (and Gligar), does more to Entei and Chandelure, and does more to Fairy types (defensive ones should be worn down if you attempt a Yanmega sweep, Air Slash does more to specs Sylveon and OHKOes Whimsicott).
I just wanna point out that Air Slash doesn't manage to OHKO BU Conk, Psychic already cleanly 2HKOes AV, Ancientpower can hit the Fire and Flyings, and LO Bug Buzz shreds offensive Whimsicott, but I suppose for the sake of target compression it's still worth it; I just thought Psychic + Ancientpower is optimal coverage for Speed Boost Yanmega.