SAS6i/r controler doesn't have any cache (opposite to PERC5 or PERC6 or the brand new ones). No cache on controler plus 7.2K RPM SATA drives can lead to bad perfs, especially under load.
In you case, "load" might not be related to the number of user but more to number of inbound/outbound messages.
Do you have lots of messages?
Is AV/AS activated on ZCS?
If so, it can be a good idea to use a ramdrive for amavis temp directory as this kills the SATA drives for sure.
RAM seems enough, your server is not swapping (as per your screenshot).
I have several servers running RAID1 only (SAS 15K, not SATA 7.2K) for OS and primary storage, and HSM on another RAID1 (SATA 7.2K), even with more than 200 users on them.
But I'm running a ramdrive for amavis (with 8 GB of RAM and a 2.23GHz QuadCore).
Here's the last mail report on one of these server:
57 deferred (598 deferrals)
65935 rejected (71%)
0 reject warnings
0 discarded (0%)
1761m bytes received
2814m bytes delivered
1998 sending hosts/domains
278 recipient hosts/domains