I would double-check your Geekbench results on the 8-core 2.93GHz vs. the 2-core X5680 chips. The 8-core 2.93GHz Mac Pro is scoring about 16,000 while the 12-core X5680 systems are scoring above 28,000!! I would definitely consider that a significant upgrade!!
2.93 GHz X5570 8-core Mac Pro (highest config now available from Apple):
3.33 GHz X5680 12-Core Hackintosh Running Mac OS X:
I think this proves the 6-core chips DO offer SIGNIFICANT improvement over the older 8-Core Nehalems.