I do like ExtremeSpeed on it.
Take this situation for example:
Zygarde is slower than your opponent's Pokemon. Zygarde uses Earthquake but does not KO the opponent and Zygarde can finish off your opponent next turn. Your opponent would attack during the next turn anyway, but you can use Coil to boost and use the boosted ExtremeSpeed to finish off your opponent.
ExtremeSpeed allows for attack chaining on faster Pokemon: take a hit, use Earthquake or Stone Edge and then ExtremeSpeed.
Take this situation for example:
Zygarde is slower than your opponent's Pokemon. Zygarde uses Earthquake but does not KO the opponent and Zygarde can finish off your opponent next turn. Your opponent would attack during the next turn anyway, but you can use Coil to boost and use the boosted ExtremeSpeed to finish off your opponent.
ExtremeSpeed allows for attack chaining on faster Pokemon: take a hit, use Earthquake or Stone Edge and then ExtremeSpeed.
ExtremeSpeed works best if you are slower.Both have their merits, but I'd go with Sub on a specially defensively set. Extremespeed has more merit if you're running an offensive 252 HP / 252+ Atk, for example.