Did Mac OS X 10.5.7 Modify Management of Multithreaded Tasks?

When the Nehalem-based Mac Pro were released, many users noticed that the single Quad Core Xeon CPU model was faster than the corresponding dual Quad Core CPU model with many applications, Adobe Photoshop included. In fact, it was a bit like if OS X was losing time in distributing work load and collecting and interpretatin results on a larger number of cores. As reported by DigLloyd, it seems that Apple modified some layers in the system to improve multreading management with Mac OS X 10.5.7:

"I had previously written When More is Less, showing that disabling half the CPUs could dramatically speed up certain Photoshop operations on a dual-CPU Mac Pro Nehalem. Well, that gap has narrowed considerably; the disabling trick still improves matters, but only by
a small amount. ... various Photoshop filter tests on my dual-CPU (16 virtual core) Mac Pro Nehalem now run a smidgen faster with all 16 virtual cores, or a lot slower with some cores disabled, as compared to 10.5.6."

One will need to run additional test to ensure that this is a general behavior for all applications, and not only for Adobe Photoshop.