In January 2012, OWC released the 2nd generation of their 16GB memory modules, which now run run at full-bus speed of 1333 MHz, and perform as well as 8GB memory modules.
According to OWC—
These 16GB modules allow full 1333Mhz bus performance and are faster in both 1066MHz and 1333Mhz memory bus Mac Pros with the Dual-Rank design than the Quad-Rank module options they replace.
I tested the 16B memory modules on both a 6-core 3.33 GHz and 12-core 3.33 GHz Mac Pro. The modules on both machines showed up as 1333 MHz, and tested at the same speed as the 8GB memory modules, using MemoryTester with the stress command, which display memcpy() speed as part of its operation.