Overclocking the Phenom II X4 940 by just 600MHz seems to have a profound negative impact on efficiency. Before any overclocking took place the Phenom II X4 940 was one of the most efficient processors tested, especially when under load.
Once overclocked this changed dramatically with a 49% power consumption increase when under load. Even at idle the system used 47% more power, which is perhaps too much for the 20% boost in frequency.
A sweet spot could have been achieved by running the Phenom II X4 940 at 3.5GHz without any voltage increases, though this may not have been enough to beat the Core 2 Quad in a majority of tests like we just showed. Meanwhile, the Core i7 920 is making a showing by offering better performance and efficiency, albeit the increased cost.