So with some help from the IRC channel (thanks macdice and JanniCash) it's come to light that my RAID1 comprised of 2 * 7200RPM disks is reporting ~500 ops/sec in pg_test_fsync.
This is higher than the ~120 ops/sec which you would expect from 720RPM disks - therefore something is lying.
Breaking up the RAID and re-imaging with JBOD dropped this to 50 ops/sec - another question but still looking like a real result.
So in this case it looks like the RAID controller wasn't disabling caching as advertised.
yup -- that's the thing. Performance numbers really tell the whole (or at least most-) of the story. If it's too good to be true, it is. These days, honestly I'd just throw out the raid controller and install some intel ssd drives.
