Unless you're running an offensive item on aegis (which you should) aegis has next to no chance of beating a full life xerneas with iron head + shadow sneak. Yveltal loves coming in on +2 aegis since it ohko's with foul play in shield form. Aegis is one of the big 4 because it beats all the others with the exception of some yveltal and considering how every team should have the big 4, having a mon that can beat them is definitely a must.I don't understand how Aegislash needs all 3 moves to beat the 'big 4' of ubers since Iron Head+Shadow Sneak is enough. SD Aegislash is supposed to be a cleaner,not a dedicated check. So SD Aegi doesn't have to beat everything. Also,there is a SpDef King's Shield+3 attacks set which is made specifically for countering Xerneas and Mewtwo.
Regarding Gyro Ball,it's an option on the SpDef set but not usable on the SD set since Iron Head is superior in every situation except against a +2 Xerneas and even if it can live an Iron Head,it gets finished off by a priority Shadow Sneak.
Gyro ball only hits +2 spd xern which has its uses admittedly.