Conclusion: To be fair, the i7-3960X isn't all that impressive when gaming either, though it's at least on par with the 2600K. I believe the SB-E parts struggle here because they have inferior memory performance to the 2500K and 2600K. Despite the newer chips supporting quad-channel RAM, the SB-E processors have a higher memory latency, which attributes for the loss in bandwidth.
Excerpt: Intel muscled into its own high-end desktop CPU territory when it launched the Sandy Bridge E (SNB-E) chips in November 2011. Expanding on the specifications of the excellent mainstream second-generation Core chips by throwing more cores, cache and memory bandwidth into the equation, SNB-E was always destined to be good. But having performance hegemony at the high end means Intel dictates release schedules as it sees fit.