More interesting than the valid FSB/DRAM Speed Configurations are the actual specifications for the X48 memory controller, that can be found in this document:http://www.intel.com/Assets/PDF/datasheet/319122.pdf
.... on the pages 18 & 19:
- Supports memory data transfer rates of 800, 1066, 1333, 1600MHz for DDR3. The
1600MHz memory support requires Intel ® Extreme Memory Profile (XMP) DDR3-
- Directly supports one or two channels of DDR3 memory with a maximum of two
DIMMs per channel. When using DDR3-1600 XMP DIMMs the X48 MCH supports
single DIMM per channel where DIMM1 is populated and DIMM0 is not populated
- I/O Voltage of 1.5V for DDR3-800, DDR3-1066, and DDR3-1333. DDR3-1600XMP
DIMMs use an I/O voltage of 1.9V.
This fact that Intel mentions "XMP", a "single DIMM per channel" population restriction and an operating voltage of 1.9V (0.4V above the industry standard specifications for DDR3) when DDR3-1600 modules are mentioned, cleary indidates that DDR3-1600 operation is to be considered an overclocked/overvolted rating which actually exceeds the native specifications.
After all, this explains why so many people run into stability problems when they are trying to operate their memory @DDR3-1600 on an X48 Chipset board.