I did a bunch of calculations earlier today and learned that even with 0 speed investment the only unboosted thing that outspeeds a Minior with Shell Smash in OU right now is Weavile with a Choice Scarf.
....
Anyway, here, I did a bunch, these are assuming you are in unarmored form and a Shell Smash used.
Another thing to keep in mind is that the armored form is immune to status effects, so no need to worry about stuff like Klefki while you're setting up.
+2 252+ Atk Minior Acrobatics (110 BP) vs. 252 HP / 172 Def Clefable: 345-406 (87.5 - 103%) -- 18.8% chance to OHKO
252+ Atk Dragonite Aqua Jet vs. 0 HP / 0 Def Minior in Sun: 74-88 (28.3 - 33.7%) -- 0.2% chance to 3HKO (Just to show Minior might work in Sun with Solar Beam)
+2 252+ Atk Minior Stone Edge vs. 0 HP / 4 Def Multiscale Dragonite: 310-366 (95.9 - 113.3%) -- 75% chance to OHKO
+2 252+ Atk Minior Acrobatics (110 BP) vs. 248 HP / 168+ Def Amoonguss: 648-764 (150.3 - 177.2%) -- guaranteed OHKO
+2 252+ Atk Minior Acrobatics (110 BP) vs. 4 HP / 252+ Def Eviolite Chansey: 435-513 (67.7 - 79.9%) -- guaranteed 2HKO
+2 252+ Atk Minior Stone Edge vs. 248 HP / 100+ Def Mega Charizard X: 456-536 (127 - 149.3%) -- guaranteed OHKO
+2 252+ Atk Minior Stone Edge vs. 0 HP / 0 Def Mega Diancie: 276-325 (114.5 - 134.8%) -- guaranteed OHKO
+2 252+ Atk Minior Acrobatics (110 BP) vs. 252 HP / 88+ Def Ferrothorn: 220-261 (62.5 - 74.1%) -- guaranteed 2HKO after Leftovers recovery (Ferrothorn can't safely switch into Minior)
+2 252+ Atk Minior Acrobatics (110 BP) vs. 0 HP / 4 Def Garchomp: 342-403 (95.7 - 112.8%) -- 75% chance to OHKO (Mega sets tank and kill us though)
+2 252+ Atk Minior Acrobatics (110 BP) vs. 16 HP / 8 Def Mega Gardevoir: 460-543 (163.7 - 193.2%) -- guaranteed OHKO
+2 252+ Atk Minior Acrobatics (110 BP) vs. 244 HP / 8 Def Gliscor: 270-318 (76.7 - 90.3%) -- guaranteed 2HKO after Poison Heal
+2 252+ Atk Minior Earthquake vs. 0 HP / 4 Def Jirachi: 396-468 (116.1 - 137.2%) -- guaranteed OHKO
+2 252+ Atk Minior Acrobatics (110 BP) vs. 0 HP / 4 Def Keldeo: 714-842 (221 - 260.6%) -- guaranteed OHKO
+2 252+ Atk Minior Stone Edge vs. 248 HP / 252+ Def Rotom-W: 205-243 (67.6 - 80.1%) -- guaranteed 2HKO after Leftovers recovery (can't switch into you)
+2 252+ Atk Minior Stone Edge vs. 252 HP / 252+ Def Skarmory: 169-201 (50.5 - 60.1%) -- 86.7% chance to 2HKO after Leftovers recovery
Also, this is funny but even with absolutely no speed investment, Minior has 1 more speed than max speed Excadrill, so Minior outspeeds and OHKOs any non-Air Balloon Excadrill, even in sand.
However, Minior is beat by Mega Slowbro and Sableye, Ferrothorn if you're not careful, Hippowdon, Tyranitar, any Azumarill, Landorus-T.
Though since you outspeed anything with shell smash, I think you can invest fully in both atk and spatk and run a mixed set. Almost nothing can switch into Minior besides stuff like Mega Aggron.
Landorus-T destroys Minior though so maybe HP Ice is gonna be a thing if it's srs since it also lets you beat Garchomp. I don't know.