The HL-15 has components that can be upgraded.
There are open PCIe slots to use another HBA card, etc.
The power supply can be upgraded depending the power budget for another PCIe card and the SSDs that be mounted via the printable cage.
The most recent post shows 5 SSDs (raid z1). I am hoping the design can be modified (or have one posted) to support 6 SSDs (raid z2).
There are other ATX style PSUs that provided more AMPs.