We have the same situation in our infrastructure.

We are using the first DSS for serving NFS Volumes to ESX, make a volume replication to the second DSS and create a snapshot of the backup volume every night.
So, I thought, the performance impact of creating snapshots is not affecting the production DSS.
But, I have not yet tested the consistency of the ESX images in the snapshot.

@To-M:
The auto-failover feature is only for ISCSI-Volumes, isn't it?

regards
Matthias