Yep. You have, for once, a very confortable amount of RAM to handle your iSCSI share. For a write test, ZFS could cache up to two transaction groups before it has to flush to disks—requiring a pause to digest if need be. 2 * 5 s * 25 Gb/s = 250 Gb, i.e. about 25 GB (at 10 bits per byte with headers, so this 1 GiB Crystal test looks like it is not stressful at all.
Having 15 clients competing for the 300 IOPS of a single raidz2 vdev, when each operation actually requires multiple drive access (for the data, for all the metadata of the filsystem in the zvol, and for all ZFS metadata on top of it) would be a lot more stressful, but to simulate that you’d have to play with appropriate options for fio.
Testing ZFS performance easily goes to 8 and above on this scale…