Ok, I've been reading these pages, but I am still a tad confused, what is the ideal Item / EV Spread for an Aegislash running King's Shield / Sacred Sword / Shadow Ball / Iron Head?
Don't use Iron Head, IMO. It's resisted by too many types, and Shadow Ball is going to be your main attack. Think of Shadow Ball as a Specs Draco Meteor. It's powerful, spammable, and only resisted by a select few mons in OU:
0 Atk Aegislash-Blade Sacred Sword vs. 252 HP / 0 Def Tyranitar: 84.1 - 100%
0 Atk Aegislash-Blade Iron Head vs. 252 HP / 0 Def Tyranitar: 56.4 - 66.8%
0 Atk Aegislash-Blade Sacred Sword vs. 252 HP / 0 Def Bisharp: 110.1 - 130.5%
0 Atk Aegislash-Blade Sacred Sword vs. 252 HP / 252+ Def Mandibuzz: 15 - 17.9%
0 Atk Aegislash-Blade Iron Head vs. 252 HP / 252+ Def Mandibuzz: 20 - 24%
0 Atk Aegislash-Blade Sacred Sword vs. 0 HP / 0 Def Mega Absol: 102.5 - 121%
0 Atk Aegislash-Blade Sacred Sword vs. 0 HP / 0 Def Greninja: 89.8 - 105.9%
0 Atk Aegislash-Blade Sacred Sword vs. 0 HP / 0 Def Hydreigon: 62.1 - 73.2%
You switch in and use either Shadow Ball, or Sacred Sword to hit the appropriate switchin, then use Shadow sneak to finish it off. Use King's Shield when needed. People insist on running Iron Head because it's STAB, or for Fairies, but there's really no need, or no room for it.
Shadow Sneak is the standard in the top analysis you mention, not Iron Head. I think it's more useful to run no speed EVs (possibly even no IVs) so you can out-slow other Aegislash rather than to outrun one specific variant of Tyranitar. Good point about the 4 SpDef stopping Genesect's SpAtk boost.
You don't need to run 252 Atk / 252 SpA on mixed sets. Aegislash wants its HP investment and King's Shield, and doesn't lose anything by running both.
Also, I...
I really disagree with everything you've said, actually. It's all wrong. Aegislash has 60 Base Speed. Tyranitar has 61. You run 12 EVs to outspeed it and KO. Tyranitar is a much more common switchin than opposing Aegislash. Why would you switch Aegislash into Aegislash? Why would you stay in? Even if that situation did happen, I think I'd rather be faster:
252 Atk Aegislash-Blade Shadow Sneak vs. 240 HP / 0 Def Aegislash-Shield: 31.7 - 38%
252+ SpA Aegislash-Blade Shadow Ball vs. 240 HP / 4 SpD Aegislash-Shield: 69.7 - 82.8%
The Speed advantage gives you a much better chance to come out on top. Instead of relying on Shadow Sneak and winning the Speed tie, I have a good chance to survive the Sneak in Shield Forme, then hit back with Shadow Ball.