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OWC 16GB Memory Modules Clock Speed (2011 variant)
The January, 2012 dual-rank modules run at 1333MHz, and can be expected to perform slightly better than what is shown here.
Discussion below applies to the 2011 rev-1 modules.
The Mac Pro motherboard clocks down the speed of 16GB modules from 1333MHz to 1066MHz, or even 800MHz if more than the optimal number is installed. This results in reduced memory bandwidth, which can impact performance with some programs, but usually has little effect— see the tests.
6-core Mac Pro @ 3.33Ghz
The Mac Pro motherboard downclocks 1333MHz memory to 1066 MHz when installing three 16GB modules. This can have a minor speed hit, see the tests for details, but usually the effect is of no consequence.
64GB does not work properly— with 4 X 16GB, Mac OS X (or perhaps the motherboard) fails to work properly. With 4 X 16GB installed, only 2GB of memory is used by Mac OS X, and at 800Mhz speed. Performance with 64GB installed is thus abysmal, with massive virtual memory page-swapping. While my testing showed that it is possible to install 3 X 16GB + 1 X 8GB = 56GB, that combination runs at 800MHz. That configuration is also not guaranteed to work by OWC.
12-core Mac Pro @ 3.33Ghz
Not a misprint— the test 12-core Mac Pro has been upgraded to run at 3.33GHz.
The Mac Pro motherboard downclocks 1333MHz memory to 1066 MHz when installing six 16GB modules, and to 800 MHz with eight 16GB modules. Since only the first 96GB is used (128GB reported), there is no point to installing 8 modules.
With 8 X 16GB installed (or 6X16GB + 2X8GB), only the first 96GB is recognized as usable, and the memory speed drops to 800 MHz. This is not an Apple (software) supported configuration; 96GB is the maximum that Mac OS X Lion will recognize, though Snow Leopard