I doubt for most distributed object stores (with consistency or availability guarantees), performance is bottlenecked by the underlying filesystem access. Probably, network latency runs the show.
To what degree that's true depends significantly on use case and architectural choices.
One design error we made with Riak in its early days was shuffling data around the servers via distributed Erlang, which led to some serious performance bottlenecks.
Distributed Erlang is better used for control messages; large blocks of data should be distributed out of band.
Nonetheless, our customers regularly needed assistance with disk tuning, because disk access does matter quite a bit.
