Currently using a RAID controller to create a Striped RAID volume (RAID 0) on two seperate Harddrives is the only way to get SSD like speeds and still benefit from large capacity storage. However, at minimum this requires atleast two Harddrives, two SATA cables, and two seperate SATA ports on the PC motherboard. The benefit of doing this is the native speed of the drives is multiplied by the amount of drives striped in RAID 0 volume.
With the new SATA Express interface technology currently being implemented on the newer Intel 9-Series Platforms, PCI express is able to interface with storage devices to achieve up to 10Gbps bandwidth on a single SATA express port. The current WD HDDs will not be able to fully take advantage of this technology without the use of an onboard SSD which is usually a fraction of the HDD capacity.
Instead of using large platters to make the current 3.5" SATA HDDs, why not use multiple 2.5" HDDs (or the parts) and insert these inside of a 3.5" HDD form factor casing? Doing this means that RAID technology may be used in an internal HDD to stripe the multiple platters/heads/HDDs. Have an internal SATA express/RAID controller and then Add an 8GB/16GB SSD for disaster recovery and BSODs.
Eventually maybe striped RAID + Redundancy can be applied to this concept for higher reliability.
This concept would probably be closest to bringing HDDs to max SATA express speed (10Gbps) without using highspeed SSDs.