12 EVs is an insignificant amount that allows you to beat a a fairly common threat (mixed SpD TTar). I don't care if you don't OHKO, you come very close in doing so (84.1 - 100%) and almost always OHKO after SR. The EVs stay, unless someone has a good reason for them to be placed elsewhere.
I know for my own personal Tyranitar, I pour everything into defenses.. a combination of Shadow Ball + Sacred Sword including Stealth Rock damage fails to KO a 252 HP / 156+ Def / 100 Sp.Def Assault Vest Tyranitar (Shadow Ball is there under the assumption that it's switching into the Shadow Ball, predicting the Ghost-type move)
0 Atk Aegislash-Blade Sacred Sword vs. 252 HP / 156+ Def Tyranitar: 272-320 (67.3 - 79.2%) -- guaranteed 2HKO after Stealth Rock (272, 272, 276, 280, 284, 288, 288, 292, 296, 300, 304, 304, 308, 312, 316, 320)
252+ SpA Aegislash-Blade Shadow Ball vs. 252 HP / 100 SpD Tyranitar in Sand: 48-57 (11.8 - 14.1%) -- possible 7HKO after Stealth Rock (48, 49, 49, 50, 51, 51, 52, 52, 53, 54, 54, 54, 55, 56, 57, 57)
The big debate for this is whether or not you want to outspeed Tyranitar, on a Quiet set for Aegislash, the only Tyranitar set you're really looking at are the negative speed sets.. so mostly the mixed sp.def set Tyranitar set, which leads to two major ones: Assault Vest or Support (I've mentioned this before, but on an Assault Vest set, a Relaxed nature > Sassy nature.. 252 HP/156+ Def/100 Sp.Def provides more stats than 252 HP/252 Def/4+ Sp.Def.. by 5 Def and 1 Sp.Def to be precise)
Against the Support set (mostly thinking of Stealth Rocks with Special Attack coverage), which would run less Def EVs, yes, the 12 Speed EVs are helpful. Though.. Quiet nature doesn't help at all against sets which run no special attacks (neutral speed) at all and the EVs get wasted completely (Aegislash requires 31 Speed IVs for this as well)
Against the Assault Vest set I proposed, you'd be playing more of a prediction game against Crunch (unless they go for safe Flamethrower/Fire Blast/Earthquake), if you outspeed and you realize they survive the Sacred Sword, then you'll be penalized heavily for it. I'd suggest two EV/IV spreads, depending on what the user needs to be done, one with 0 speed (the 12 Speed goes into 12 Att, which raises the Sacred Sword damage)
0 Att EVs:
http://puu.sh/6deXQ.png
12 Att EVs:
http://puu.sh/6deZ7.png
Though.. the more I think about it, 31 Speed EVs along with 12 Speed EVs definitely wins one possibility and has a possibility of being punished in the other.. while the other one would end up taking damage (or allow Stealth Rocks to be setup), but can end up dealing with the second scenario without as much risk (in an isolated 1v1 situation that is.. not going to really go too much into theorymon for this). Hmm.. the more I think about it though, the more I feel like definitively winning first scenario outweighs winning the second scenario better.