Looks to me like the (severely) limiting factor there is the SATA speed.
This is the big downside to doing RAID 0 "in the box" rather than taking the conventional approach. Backing this up, I see that OCZ also do a 1TB SSD with internal RAID 0 as a PCIeX8 card in their "Z-Drive" range and that apparently kicks out 870/780 on Reads/Writes with sustained write at 600.
Not too much more expensive either. Let's face it, if speed's important enough that you can justify chucking over two grand at a disk, you can probably run to a few hundred more for that sort of performance hike.