This isn't really relevant, since Mega Gengar was banned from OU months ago.
While true, being able to outspeed Base 130's is an important tier, able to outspend Greninja, Landorus, Thundurus, Keldeo, Terrakion, Mega Pinsir (forces Quick Attack over Earthquake). It's bulky enough to take their best option using only HP investment, and its natural Sp. Attack is enough to beat them. It can even set up Agility on some of them.
0 SpA Mold Breaker Mega Ampharos Thunderbolt vs. 4 HP / 0 SpD Thundurus: 181-214 (60.3 - 71.3%) -- guaranteed 2HKO after Leftovers recovery
252 SpA Life Orb Thundurus Hidden Power Ice vs. 252 HP / 0 SpD Mega Ampharos: 153-182 (39.8 - 47.3%) -- guaranteed 3HKO
252 SpA Life Orb Thundurus-T Hidden Power Ice vs. 252 HP / 0 SpD Mega Ampharos: 172-203 (44.7 - 52.8%) -- 22.7% chance to 2HKO
Thundurus loses one on one even with LO, and even Therian can't guarantee the win, since it loses Volt Absorb to Mold Breaker.
0 SpA Mold Breaker Mega Ampharos Hidden Power Ice vs. 4 HP / 0 SpD Landorus: 324-384 (101.2 - 120%) -- guaranteed OHKO
252 SpA Life Orb Sheer Force Landorus Earth Power vs. 252 HP / 0 SpD Mega Ampharos: 421-499 (109.6 - 129.9%) -- guaranteed OHKO
Both get the OHKO, but Landorus becomes manageable if it HAS to use Earth Power to not die. Becomes irrelevant if Ampharos has an Agility boost.
0 SpA Mold Breaker Mega Ampharos Thunderbolt vs. 4 HP / 0 SpD Greninja: 398-470 (139.1 - 164.3%) -- guaranteed OHKO
0 SpA Mold Breaker Mega Ampharos Focus Blast vs. 4 HP / 0 SpD Greninja: 354-418 (123.7 - 146.1%) -- guaranteed OHKO
252 SpA Life Orb Greninja Ice Beam vs. 252 HP / 0 SpD Mega Ampharos: 304-359 (79.1 - 93.4%) -- guaranteed 2HKO
Greninja can't manage a OHKO without hazards, and Ampharos has an option if it changes types for the SE attack.
0 SpA Mold Breaker Mega Ampharos Thunderbolt vs. 4 HP / 0 SpD Mega Pinsir: 330-390 (121.3 - 143.3%) -- guaranteed OHKO
252 Atk Mega Pinsir Earthquake vs. 252 HP / 0 Def Mega Ampharos: 238-282 (61.9 - 73.4%) -- guaranteed 2HKO
252 Atk Aerilate Mega Pinsir Quick Attack vs. 252 HP / 0 Def Mega Ampharos: 46-55 (11.9 - 14.3%) -- possible 7HKO
252 Atk Pinsir Earthquake vs. 252 HP / 0 Def Mega Ampharos: 204-242 (53.1 - 63%) -- guaranteed 2HKO
Pinsir needs Max damage rolls and SR to win, and even without Mega Evolving, T-bolt is still a 2HKO.
0 SpA Mold Breaker Mega Ampharos Thunderbolt vs. 0 HP / 4 SpD Keldeo: 326-386 (100.9 - 119.5%) -- guaranteed OHKO
252 SpA Choice Specs Keldeo Secret Sword vs. 252 HP / 0 Def Mega Ampharos: 199-235 (51.8 - 61.1%) -- guaranteed 2HKO
With SS being its strongest option, even Specs Keldeo is checked by Mega Ampharos
0 SpA Mold Breaker Mega Ampharos Focus Blast vs. 0 HP / 4 SpD Terrakion: 292-344 (90.4 - 106.5%) -- 37.5% chance to OHKO
252+ Atk Life Orb Terrakion Earthquake vs. 252 HP / 0 Def Mega Ampharos: 296-351 (77 - 91.4%) -- guaranteed 2HKO
Checks LO Terrakion, especially with Spikes or at least SR
Granted, it can't consistently beat them all without Wish support, but Ampharos is more than effective enough to check a lot of fast threats in the current Meta game. I left out EVs beyond HP to show how versatile you can be investing. You can forgo a coverage move for Agility to outspeed unbolted opponents and clean up, Sp. Attack to net a few more KOs, or either defensive stat to better check the aformeated threats, maybe even counter some of them, like Keldeo.
252 SpA Choice Specs Keldeo Secret Sword vs. 252 HP / 252+ Def Mega Ampharos: 144-171 (37.5 - 44.5%) -- guaranteed 3HKO
Mega Ampharos is a surprisingly bulky and effective attacker against quite a few of the higher viability offensive threats, if you can spare your Mega slot.