I spent most of today recoding diglloydTools to be able to characterize memory and computing performance with 16-core machines, like the new Mac Pro (8 real cores, 16 virtual cores).
When is more memory slower? When it drops DDR3 memory speed from 3X (triple-channel) to 2X, which is apparently what installing 8 modules instead of 6 does in the new Mac Pro “Nehalem”. It apparently downgrades speed for all modules, not just the last two. This is preliminary based on numbers provided to me by Rob-Art at barefeats.com on his 2.26GHz MacPro using my just-revised diglloydTools.
See my MacPerformanceGuide writeup for details.