That throws all kinds of things out of whack and would require a major overhaul of our ideas of what Stat Switch is like.
Also it makes Azumarill ultra-broken.
Stat Switch Speed tiers
I'm focusing on things that break 100, since the Mew-alikes are all unchanged and indeed a lot of Pokemon in the vicinity of 100 are still the same, and nobody cares about the million-and-one things in the bottom.
Shuckle, Mega Aggron, Mega Steelix: 230 Speed.
Regirock, Regice, Steelix,: 200 Speed.
Aggron, Mega Slowbro, Primal Kyogre, Primal Groudon: 180 Speed.
Mega Garchomp: 170 Speed.
Bastiodon: 168 Speed.
Mega Ampharos, Mega Banette: 165 Speed.
Mega Tyranitar: 164 Speed.
Snorlax, Mega Diancie: 160 Speed.
Ho-oh: 154 Speed.
Carbink, Diancie, Doublade, Probopass, Registeel, Dialga, Giratina-Altered, Giratina-Origin, Reshiram, Zekrom, Mega Swampert, Kyogre, Groudon: 150 Speed.
Cofagrigus, Mega Metagross, Mega Camerupt: 145 Speed.
Forretress, Lairon, Rhyperior, Wigglytuff, Conkeldurr, Darmanitan-Zen: 140 Speed.
Nosepass, Dusknoir, Metagross, Mega Blastoise: 135 Speed.
Tyranitar, Dragonite: 134 Speed.
Mega Abomasnow: 132 Speed.
Heatran, Sylveon, Golem, Machamp, Ursaring, Lapras, Magnezone: 130 Speed.
Mega Audino: 126 Speed.
Reuniclus, Dusion, Beeheeyem, Armaldo, Crustle, Mega Mawile (note that it holds onto its Attack!), Tangrowth, Exeggutor, Omastar, Articuno, Mega Sableye: 125 Speed.
Golurk, Pangoro: 124 Speed.
Dragalge, Aurorus, Emboar, Mega Venusaur: 123 Speed.
Super-size Gourgeist, Large-size Gourgeist, Chesnaught: 122 Speed.
Magcargo, Kecleon, Granbull, Druddigon, Donphan, Crawdaunt, Clawitzer, Wheezing, Luxray: 120 Speed.
Hippowdon: 118 Speed.
Bronzong: 116 Speed.
Eelektross, Sudowoodo, Ampharos, Cacturne: 115 Speed.
Gorebyss: 114 Speed.
Gastrodon, Empoleon: 111 Speed.
Slowbro, Slowking, Marowak, Vileplume, Lickilicky, Beartic, Bouffalant, Trevenant, Swampert, Walrein, Mega Altaria: 110 Speed.
Torterra: 109 Speed.
Spiritomb, Samurott: 108 Speed.
Cradily: 107 Speed.
Maractus: 106 Speed.
Sunflora, Wormadam (all three Forms), Camerupt, Gurdurr, Octillery, Muk, Huntail, Porygon2, Jellicent, Heatmor, Seismitoad, Blastoise, Feraligatr, Mesprit: 105 Speed.
Audino: 103 Speed.
Vespiquen: 102 Speed.
Aromatisse: 101 Speed.
Omanyte, Azumarill, Machoke, Carnivine, Piloswine, Ludicolo, Metang, Politoed, Venusaur, Meganium: 100 Speed.
Pokemon that retain their 100+ Speed in Stat Switch.
Deoxys Attack Forme, Mega Alakazam: 150 Speed.
Mega Beedrill: 145 Speed.
Mega Mewtwo Y: 140 Speed.
Mega Mewtwo X, Mewtwo, Mega Gengar: 130 Speed.
Darkrai: 125 Speed.
Mega Pidgeot: 121 Speed.
Arceus, Alakazam, Mega Salamence: 120 Speed.
Azelf, Mega Absol, Mega Houndoom, Mega Rayquaza: 115 Speed.
Mega Lucario: 112 Speed.
Thundurus-Incarnate, Tornadus-Incarnate: 111 Speed.
Archeops, Latios, Latias, Lugia, Espeon, Gengar, Mega Gallade: 110 Speed.
Durant: 109 Speed.
Keldeo, Terrakion, Virizion, Cobalion: 108 Speed.
Pyroar: 106 Speed.
Mienshao, Cryogonal, Zoroark, Mega Pinsir, Scyther, Kadabra, Mega Sharpedo: 105 Speed.
Delphox: 104 Speed.
Garchomp: 102 Speed.
Landorus-Incarnate, Thundurus-Therian: 101 Speed.
Volcarona, Victini, Mew, Celebi, Jirachi, Manaphy, Shaymin-Land, Regigigas, Palkia, Staraptor, Salamence, Slaking, Mega Gardevoir, Mega Blaziken, Entei, Miltank, Typhlosion, Zapdos, Mega Kangaskhan, Dodrio, Tentacruel, all variants on Charizard, Mega Glalie: 100 Speed.
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Worth noting is that 100, 105, 110, and oddly enough, 125 are fairly crowded Speed tiers. Also worth noting is there isn't an actual Speed king in this meta -unless you take into account that Shuckle is the only Pokemon in its tier that isn't a Mega and therefore gets it on the first turn.
Also worth noting is that Porygon2 is actually faster than Porygon-Z in this meta -and in fact you're probably better off using Porygon2 in general, unless you think Adaptability is so good, because Eviolite Porygon2 laughs in the face of Porygon-Z having shifted its Special Attack to Defense because Porygon2 gives up no defenses (Its Special Attack is its highest stat just like Porygon-Z) while having identical Attack and only 10 less Special Attack, so unless you're valuing Adaptability heavily... just run Porygon2 with Physical attacks and skip Porygon-Z.