Even if this does not look like good numbers, I would like you to analyze more data, maybe it is the max that your hardware can do.
One thing you can do is to look at
iostat to see what the utilization of the disk is. If it's being heavily used, then that's probably just as fast as it can go. If this is not used,..., we need to continue the investigation.
I would recommend first increasing the incoming writes to confirm that batching is a strong culprit. And then increasing the workers to 6 should double that rate. I wouldn't recommend going to 8 on this system since he's only got 8 cores and should leave ~4 of them for erlang/memcached/etc.