I think there's potential here. Deoxys-Speed's main niche over other stallers like Mega Slowbro is it's huge Speed (duh) and potential for an offensive set, making it rather unpredictable. However, it's biggest fault compared to Slowbro and co is it's utter lack of natural bulk. Deoxys-Speed possesses a pitiful base 50 HP, and unlike Deoxys-Defense, it's defenses are not high enough to compensate for it. It has rely on prediction against set-up mons like Landorus-T (At least, the stall set does) and mixed attackers, and scarfers absolutely ruin it. And as you said, in order to outspeed +1 base 100s, it needs to run much more Speed, sacrificing needed bulk. That said, it looks like a nifty alternative for those who miss Deoxys-Defense. But we'll just wait and see.
Now I actually want to run some calcs and get a better idea of what this beats.
It has rely on prediction against set-up mons like Landorus-T
252 Atk Landorus-Therian Tectonic Rage (180 BP) vs. +2 252 HP / 80 Def Deoxys-Speed: 160-189 (52.6 - 62.1%) -- 98.8% chance to 2HKO after Leftovers recovery
252 Atk Landorus-Therian Earthquake vs. +2 252 HP / 80 Def Deoxys-Speed: 90-106 (29.6 - 34.8%) -- 100% chance to 4HKO after Leftovers recovery
You can safely Iron Defense against Lando turn one, and if they click SD twice, you can Taunt the third time. Point is, you outspeed and can always have the stat advantage for when Lando attacks. Once this happens, you can recover off damage, keep it Taunted, and stall out EQs. The same holds true for most enemy stat boosters.
Mixed attackers of the tier:
Kyurem-Black
252 SpA Teravolt Kyurem-Black Ice Beam vs. 252 HP / 124 SpD Deoxys-Speed: 133-157 (43.7 - 51.6%) -- guaranteed 3HKO after Leftovers recovery
252 Atk Teravolt Kyurem-Black Outrage vs. +2 252 HP / 80 Def Deoxys-Speed: 120-142 (39.4 - 46.7%) -- guaranteed 3HKO after Leftovers recovery
252+ Atk Teravolt Kyurem-Black Subzero Slammer (200 BP) vs. +2 252 HP / 80 Def Deoxys-Speed: 220-259 (72.3 - 85.1%) -- guaranteed 2HKO after Leftovers recovery
You can comfortably lead with Iron Defense. If a Scarf Kyu-B is clicking Ice Beam, you can Amnesia the next turn and stall it out with Recover.
Tapu Koko
252+ SpA Tapu Koko Gigavolt Havoc (185 BP) vs. +2 252 HP / 124 SpD Deoxys-Speed in Electric Terrain: 192-226 (63.1 - 74.3%) -- guaranteed 2HKO after Leftovers recovery
4 Atk Tapu Koko Wild Charge vs. 252 HP / 80 Def Deoxys-Speed in Electric Terrain: 165-195 (54.2 - 64.1%) -- guaranteed 2HKO after Leftovers recovery
4 Atk Tapu Koko Wild Charge vs. +2 252 HP / 80 Def Deoxys-Speed in Electric Terrain: 84-99 (27.6 - 32.5%) -- 62.5% chance to 4HKO after Leftovers recovery
You can lead comfortably with Amnesia. Look at that thing tank gigavolt no problem! Even mixed Wild Charge isn't an issue if you ID turn 2.
Aegislash
252+ SpA Aegislash-Blade Never-Ending Nightmare (160 BP) vs. +2 252 HP / 124 SpD Deoxys-Speed: 306-362 (100.6 - 119%) -- guaranteed OHKO
Yeah he kills you. Maybe with Z-Cosmic you have a chance, but not ID/Am.
Blaziken-Mega
252+ SpA Blaziken-Mega Blast Burn vs. +2 252 HP / 124 SpD Deoxys-Speed: 129-153 (42.4 - 50.3%) -- guaranteed 3HKO after Leftovers recovery
252 Atk Blaziken-Mega Stone Edge vs. 252 HP / 80 Def Deoxys-Speed: 128-151 (42.1 - 49.6%) -- guaranteed 3HKO after Leftovers recovery
Turns out you can comfortably Amnesia to tank a potential Blast Burn or Overheat turn one, then patch up your Defense turn 2 for Stone Edge/HJK.
Lando-I
252 SpA Life Orb Sheer Force Landorus Earth Power vs. +2 252 HP / 124 SpD Deoxys-Speed: 109-130 (35.8 - 42.7%) -- 92.8% chance to 3HKO after Leftovers recovery
4 Atk Life Orb Sheer Force Landorus Rock Slide vs. 252 HP / 80 Def Deoxys-Speed: 112-133 (36.8 - 43.7%) -- 99.1% chance to 3HKO after Leftovers recovery
Lead with Amnesia and you're fine even if he spams Rock Slide.
Life Orb Mixtini
252+ Atk Life Orb Victini V-create vs. +2 252 HP / 80 Def Deoxys-Speed: 175-208 (57.5 - 68.4%) -- guaranteed 2HKO after Leftovers recovery
4 SpA Life Orb Victini Overheat vs. 252 HP / 124 SpD Deoxys-Speed: 175-207 (57.5 - 68%) -- guaranteed 2HKO after Leftovers recovery
--
96 Atk Life Orb Victini V-create vs. +2 252 HP / 80 Def Deoxys-Speed: 140-165 (46 - 54.2%) -- 4.7% chance to 2HKO after Leftovers recovery
160 SpA Life Orb Victini Overheat vs. 252 HP / 124 SpD Deoxys-Speed: 204-242 (67.1 - 79.6%) -- guaranteed 2HKO after Leftovers recovery
You can pretty much just ID on the V-Create and Recover on the Overheat (if any Victini still run mixed). This is less of a threat than standard Band Victini.
Are there any other relevant mixed attackers? I'm not counting stuff like Psyshock because though it does hit the opposite defense, DeoS resists and never really has to worry about it.
...and scarfers absolutely ruin it.
Scarfers, you say?
Porygon-Z
252+ SpA Adaptability Porygon-Z Hyper Beam vs. +2 252 HP / 124 SpD Deoxys-Speed: 178-210 (58.5 - 69%) -- guaranteed 2HKO after Leftovers recovery
Nope.
Kyurem-Black
252+ Atk Teravolt Kyurem-Black Outrage vs. +2 252 HP / 80 Def Deoxys-Speed: 132-156 (43.4 - 51.3%) -- guaranteed 3HKO after Leftovers recovery
Nope.
Tapu Lele
252 SpA Tapu Lele Shadow Ball vs. 252 HP / 124 SpD Deoxys-Speed: 168-198 (55.2 - 65.1%) -- guaranteed 2HKO after Leftovers recovery
252 SpA Tapu Lele Shadow Ball vs. +2 252 HP / 124 SpD Deoxys-Speed: 84-100 (27.6 - 32.8%) -- 76.6% chance to 4HKO after Leftovers recovery
Nope.
Lando-T
252+ Atk Landorus-Therian Earthquake vs. +2 252 HP / 80 Def Deoxys-Speed: 97-115 (31.9 - 37.8%) -- 0.3% chance to 3HKO after Leftovers recovery
Nope.
Genesect?
252+ SpA Genesect Bug Buzz vs. +2 252 HP / 124 SpD Deoxys-Speed: 146-174 (48 - 57.2%) -- 34% chance to 2HKO after Leftovers recovery
Modest Scarf is still outsped by Deo-S. Specs is actually the best way around Deo, as well as the best Gene set overall.
Garchomp
252 Atk Garchomp Outrage vs. 252 HP / 80 Def Deoxys-Speed: 196-232 (64.4 - 76.3%) -- guaranteed 2HKO after Leftovers recovery
252 Atk Garchomp Outrage vs. +2 252 HP / 80 Def Deoxys-Speed: 99-117 (32.5 - 38.4%) -- 2% chance to 3HKO after Leftovers recovery
Jolly Garchomp is one of the few able to 2hko, by barely outspeeding DeoS and dealing just enough damage.
Kartana
252 Atk Kartana Leaf Blade vs. 252 HP / 80 Def Deoxys-Speed: 189-223 (62.1 - 73.3%) -- guaranteed 2HKO after Leftovers recovery
252 Atk Kartana Leaf Blade vs. +2 252 HP / 80 Def Deoxys-Speed: 94-112 (30.9 - 36.8%) -- guaranteed 4HKO after Leftovers recovery
Kartana is also able to 2hko most of the time. Notably, both Kartana and Garchomp lose to a Counter from Deoxys-Speed if you feel like running that over ID.
Jirachi
252 Atk Jirachi Iron Head vs. +2 252 HP / 80 Def Deoxys-Speed: 55-66 (18 - 21.7%) -- possible 7HKO after Leftovers recovery
Maybe if you get really really lucky you can beat Deo-S.
There are probably a couple more relevant scarfers, but I'm too tired to keep running calcs tonight.
TO SUM UP:
Setup attackers, Mixed attackers, and Scarfers (with a couple exceptions) are much less of a threat to Deoxys-Speed than previously thought. The best counters are wallbreakers that can nail it super effectively (Z-Aegislash, Specs Greninja, Band/Specs Pheromosa, Specs Genesect, etc.) or outspeed and 2hko through a single defense boost (scarf Garchomp/Kartana). Toxic, Taunt/Z-Taunt, high crit rate moves, etc are also good ways to deal with Deoxys-Speed.