Newer Mac processors have (Quad-Core Intel Xeon “Nehalem”) have “Virtual Cores” which “which allows two threads to run simultaneously on each core.”

This raised questions like, “do these virtual cores run at half the speed (or worse because of scheduling overhead)? If I have the option, should I set my appliction (GFS, an ABM, etc.) to use 8 real cores or 16 virtual ones?”

Go with 16. I couldn’t find an answer online, but I ran a performance test which showed that 16 virtual cores gets about 1.5 times the calculations done as 8 cores.