I decided to do an enhanced secure erase and re-install Windows 7. I ran the benchmark again (this time before installing any new drivers) and I was still seeing low write speeds. I tried a couple of newer drivers for the Intel SATA controller which also seemed to have no effect. Eventually I gave up and just started installing the rest of my drivers and software. I ran the benchmark after every couple of installations just to check it didn't suddenly get better or worse. The strange thing is that it seemed to start getting gradually better. Its almost as if the drive has some housekeeping it needs to do and it chooses to do it when its loaded with large writes. Also whatever housekeeping it is apparently can't be avoided by doing an enhanced secure erase. Maybe this is because the enhanced secure erase only generates new encryption keys and doesn't actually erase the memory cells? If this theory is correct then is there any way to get the drive to actually get this housekeeping over and done with? The optimizer doesn't seem to do it - it runs and completes apparently successfully in just a few seconds. I suspect that the real work is being put off. Maybe this is all a deliberate strategy to eliminate wear? That would make some kind of sense except why do 64KB writes still seem to happen at full speed?
I thought I should update this thread just in case anyone stumbles across it. It would appear that what I've been seeing has nothing to do with the drive itself and is caused by processor power management on my laptop. I thought that having the laptop plugged in would be sufficient but I actually had to switch from the "Balanced" profile to the "High Performance" to get the full performance of this drive in the ATTO benchmark. Other benchmarks might behave differently. I still only get about 10MB/s write and 13MB/s read on 512 byte transactions but I'm assuming this is because my CPU isn't particularly powerful. Sorry if I've wasted anyone's time.