http://www.smogon.com/dex/xy/pokemon/volcarona
As you can see on the OU Bulky Quiver Dance set, the proposed EV spread uses a Bold nature and enough Speed EVs to outspeed neutral-natured base 150 (like Modest M-Alakazam or Adamant M-Aerodactyl), and the leftover EVs are placed in HP (248, allowing 3 switch-ins with SR) and Defense (136). When using this nature and EV spread Volcarona reaches 373 maximum HP, 220 Defense and 267 Speed.
I made some calculations, and there is a better spread to use: Timid, 248 HP / 232 Def / 28 Spe. When using this spread, Volcarona reaches the same Speed benchmark than the previous set, but its Defense is 4 points higher (224 instead of 220)
To see the difference:
I think the spread should be changed
As you can see on the OU Bulky Quiver Dance set, the proposed EV spread uses a Bold nature and enough Speed EVs to outspeed neutral-natured base 150 (like Modest M-Alakazam or Adamant M-Aerodactyl), and the leftover EVs are placed in HP (248, allowing 3 switch-ins with SR) and Defense (136). When using this nature and EV spread Volcarona reaches 373 maximum HP, 220 Defense and 267 Speed.
I made some calculations, and there is a better spread to use: Timid, 248 HP / 232 Def / 28 Spe. When using this spread, Volcarona reaches the same Speed benchmark than the previous set, but its Defense is 4 points higher (224 instead of 220)
To see the difference:

I think the spread should be changed