[vegardx]

I think that depends a lot on how the data is going to be used. It sounds like you're not really using EBS volumes for what they're great at; Durability.

While instance storage is ephemeral nothing really stop you from using it as a local cache in a clustered filesystem. If you have a somewhat read intensive workload then you might see performance close to matching that of using instance storage directly.

There are some fundamental limits to how fast a clustered filesystem can be, based on things like network latency and block size. Things like locking is an order of magnitude slower on a clustered filesystem compared to locally attached storage.