From what I can see, the defensive EVs don't help Kyurem-W much. The haban berry bears most of the attacks, turning almost all Dragon type attacks that would OHKO into 2KOs (excluding choice specs/band dragons and LO Draco Meteors coming off of 150 Sp.A). However, the Haban berry only works on the first hit, so none of these moves can become 3KOs, even with your suggested special investment. However, the specially defensive investment would help against non-dragon special attackers such as Kyogre, as the scarfed form only deals 29.42 - 34.66% against specially defensive Kyurem-W with Water Spout, as opposed to 38.1 - 45.01% against Kyurem-W with no specially defensive investment. This turns a 2KO with Stealth Rock into a 3KO with Stealth Rock, and if Stealth Rock is up on their side, you can 2KO in return with Draco Meteor 98% of the time. If you instead run 252 Sp.A and a timid nature, Kyurem-W would still be unable to OHKO Kyogre, and would also risk being 2KOd in return, losing the encounter due to Kyogre's higher speed. On the other hand, you lose the ability to OHKO several pokemon, such as Latias. With 252 Sp.A and a Timid nature, Kyurem-W will OHKO Latias holding Soul Dew (with an EV spread of 252 HP / 0 Sp.D) 62.5% of the time with Draco Meteor if Stealth Rock is up, while with 0 Sp.A and a Modest nature, this drops to a mere 25%. Also, you would lose out on outspeeding max speed base 90s, which is one of the main advantages of Kyurem-W over the base 90 pokemon. This would mean that they would get more hits against you before you can KO them, making the defensive investment almost useless as unless you can OHKO them, the base 90 dragons that run max speed would always 2KO you with Draco-Meteor or Dragon Pulse(assuming 252 Sp.A and a timid nature). While the idea would be quite viable, I think that the Haban berry would be making a bigger difference than the EV spread and you would have to take into account the fact that for the things you gain defensively, you would lose something offensively as well. Edit: It also seems that you are effectively trying to turn Kyurem-W into Dialga. Dialga does not need the Haban berry, as it takes dragon type hits neutrally, and though Kyurem-W actually has better special defense than dialga, it doesn't have the typing to back it up. In my opinion, the only reason that you should be using Kyurem-W is for its base 170 Special Attack and its base 95 Speed. Lowering these two stats removes the purpose of using Kyurem-W as with your set base 90 speed pokemon with 150 Special attack will outspeed it and still hit almost as hard, having just 14 Sp.A lower than Kyurem-W if they have an EV spread of 252 Sp.A / 252 Spd with a Timid nature.