But "faster" is not always the way to measure things.
In this topic, I think that raw disk access speed is not a critical value; I've tested our setup with maildir on NFS and performance is close enough to local disks. Given this, I prefer the extra features our NAS/SAN setup gives us, like having data replication between phisically separated disks in a couple of clicks ,redundant paths to data, easier management, ...
In my test local storage was always ~ 2 times faster than nfs.
if you have the deep pockets to buy all the storage gear and replication licenses/maintainence then it's for sure far better. But if you are an isp, you have to sell the storagespace for pennies. So we came up with a cheap robust and scalable storage solution special to our mailsetup. It Supports HA with blocklevel-replication and snapshots with lvm so it's even some what manageable.
Another point against nfs is that i wanted to avoid potential locking/coruption problems. I know it can be solved by always redirecting the same user to the same server, but then again i loose the convenience of nfs. So i ditch nfs complety and use multiple storage servers in pairs running dovecot.