Why not eight cores?
Before we proceed to the benchmarks one question needs to be answered first. Why did Intel activate only six out of eight cores in the Sandy Bridge-E processors? The official explanation is that with a 130 watt TDP limit the preference goes to higher clock frequencies rather than two additional cores.
There is something to be said for this line of reasoning. The 130 W TDP limit could be maintained with eight active cores, but the clock frequency would need to be decreased to compensate. Multi-threaded applications could in theory be faster with eight cores, but in applications that aren't scalable there would be a performance decrease. The recently introduced AMS Bulldozer processor proved what the result is of an eight-core processor where each individual core is slower. For Intel it was essential that Sandy Bridge-E was faster in all situations compared to the current generation six-core Core i7 9xx processors. The only way to currently ensure this is by disabling two cores.
The question remains however why Intel has imposed a 130 W TDP on itself. The target users for this type of processor won't be offended by an additional 10 or 20 watts. Why not increase the limit to 150 watts? Intel claims that it would be too challenging in terms of validation. Standard motherboard lay-outs, CPU coolers, and cases would all need to go through the official validation process.
This is true of course, but you could argue that "where there's a will, there's a way." If Intel had had any competition on the processor market, the eight-core version of Sandy Bridge-E would perhaps have been developed. Intel's only competitor for x86 processors, AMD, concluded years ago that it could not keep up with Intel in the high-end market. Intel therefore only competes against itself. The Core i7 990X could have remained the world's fastest processor, with a side note that the more affordable Core i7 2600K was the more appealing choice for high-end consumers.
Will we ever see an eight-core version of Sandy Bridge-E? It doesn't appear on any Intel road map, so chances are very slim. But it's nice to know that the option is there at least.