252+ Atk Life Orb Absol Sucker Punch vs. 0 HP / 0 Def Shaymin: 187-222 (54.83 - 65.1%) -- guaranteed 2HKO (130 atk+life orb)
252+ Atk Mega Absol Sucker Punch vs. 0 HP / 0 Def Shaymin: 160-189 (46.92 - 55.42%) -- 69.53% chance to 2HKO (150 atk)
Yeah...no. Unless you want to go mixed i guess, but at that point youre better off with something else.
You did a damage calc against Shaymin. Shaymin! Something that's neutral to Sucker Punch and fairly bulky even without investment. Sure, it's weaker than LO Absol by a bit, but also consider that it has 115 base Speed, 115 base Special Attack, and
Magic Bounce. You're looking too much on the physical end, while that increased Special Attack allows it to use some moves in its special movepool. Check this out (This factors in the Fire Blast nerf):
0 SpA Life Orb Absol Fire Blast vs. 224 HP / 32 SpD Skarmory: 208-247 (63.6 - 75.53%) -- guaranteed 2HKO after Stealth Rock
0 SpA Mega Absol Fire Blast vs. 224 HP / 32 SpD Skarmory: 228-270 (69.72 - 82.56%) -- guaranteed 2HKO after Stealth Rock
If you ask me, having Special attacks slightly stronger than LO Absol's, while also having far more Speed and
Magic Bounce, is pretty big. That Speed lets it outrun several key threats like Terrakion, Lati@s, Jirachi, etc. Yeah, you try using any non-Scarf Terrakion to beat Absol when Mega Absol outspeeds and obliterates it with Superpower. Oh, you tried to go for Substitute or something with Latias to dodge Sucker Punch? Nope. Mega Absol outspeeds you and can OHKO you with Night Slash with just a bit of prior damage. Not to mention that Magic Bounce makes Sucker Punch more effective anyway; Mega Absol can also more effectively use moves like Fire Blast and Ice Beam to cover the likes of Skarmory, Ferrothorn (fuck Superpower Atk/Def drops,) Gliscor, Hippowdon, and Landorus-T. It's definitely a better Magic Bounce user than Espeon, that's for sure...
So, while Mega Absol won't be the best Mega Evo, it will have a niche.