firestorm + faster cast speed

Does faster cast speed have huge impact on firestorm skill?

Faster casting support gem does exactly what it says it does.

To be more specific, when your character casts a spell, it take some amount of time to cast the spell. With faster casting support you cast the spell faster. Whether or not that is a good decision for your build/playstyle is up to you. Some spells (like firestorm) cast too slow for my taste, so faster cast speed is desirable (to me) in those cases.

Edit: typo

Faster casting allows you to stack firestorms on a target as it allows you to cast a second firestorm before the previous firestorm duration has expired, so yep, if you're the kind who likes to stack firestorms, Faster Casting + Incr. Duration on Firestorm is a nice combo