I don't think a pure physical DD Latios would be effective. If DDLatios has 1 special move to beat physical walls, then what defense would most likely be sacrificed? Defense means priority users with Bullet Punch or Ice Shard would do massive damage. Sacrificing Sp.Def means Latios has less things to set up on and the Scarf-Skymin mentioned above do even more damage.
The problem is not which defense to lower, but rather, what Special Move do you pick?
Assume a spread of 252 Attack / 192 Speed / 68 Special Attack Naive (enough Speed to outrun Garchomp before a DD and Scarfed Garchomp after a DD, maxed attack because he really doesn't have attack to spare with only 90 base, and the rest in Special attack, -SpD nature because there's a lot more physical priority). Moveset is Dragon Dance, Outrage, Earthquake, <Special Attack>. Common Pokemon that this will have to contend with include Hippowdon, Skarmory, and Bronzong.
Thunderbolt versus 252/252+ Skarmory: 64-76%
Thunderbolt versus 252/252+ Bronzong: 23-27%
HP Fire Versus 252/252+ Skarmory: 47-56%
HP Fire Versus 252/252+ Bronzong: 34-40%
HP Fire Versus 252/88 Hippowdon: 22-26%
Surf Versus 252/252+ Skarmory: 32-38%
Surf Versus 252/252+ Bronzong: 23-27%
Surf Versus 252/88 Hippowdon: 60-71%
Draco Meteor versus 252/252+ Skarmory: 35-41%
Draco Meteor versus 252/252+ Bronzong: 25-30%
Draco Meteor versus 252/88 Hippowdon: 72-85%
As you can see, Bronzong is strong enough to weather at least two hits of anything Latios can throw at him, and at least three hits of anything besides HP Fire. Hippowdon requires either Draco Meteor or Surf, either of which do laughable damage to Skarmory, who requires Thunderbolt or HP Fire to 2HKO (and HP Fire can miss the 2HKO even with SR up, assuming Skarmory is carrying leftovers and not Shed Shell). Specially defensive Forretress takes these hits about as well as Skamory does, only takes half as much from Tbolt and twice as much from HP Fire. Scizor actually has better HP and Special Defense than Skarmory does, though will likely run less of an investment in Special Defense so it can still pack a punch, but again takes neutral from Tbolt and necessitates HP Fire to be even 2HKO'd, while he can launch back devastating Bullet Punches in return and has the possibility of Pursuit up his sleeve. Specially defensive Metagross paints about the same picture, only with even more natural bulk. Things like Gliscor, Cresselia, Donphan, and Suicune all have the physical bulk to easily take a +1 LO Outrage, and require either Draco Meteor or a custom-tailored special move to be able to even 2HKO them (and forget about it on Cress, even a 252/0 Cress takes a max of 55% from Draco Meteor and 42% from Shadow Ball).