So, traditional databases can be set up on a RAID 0 with multiple volumes to increase throughput but I think there will likely be a trade off with SiaCoin depending on how the hosts are set up that the data is stored.
For fault tolerance, up to 7 of the 21 hosts could fail and data would still be recoverable. So the trick would seem to be judicious in your host selection so that they meet whatever requirements there may be.
Technically, one could select all nodes that have their host db saved to such an array but, due to the host's being remote, one would still have to account for network latency.
Even so, think this could find it's own niche as a turnkey web-accessible database solution secured by blockchain cryptography.