I think what you are seeing is the bclk overclock which can go from 100 to perhaps as high as 106 or so. That is a 6% improvement.
when applied to the 1 core turbo of 3.7, you can actually get one core near that 4.1 number.
All 4 cores are limited to 3.8 The chart is an excerpt from the anandtech sandy bridge preview. It was not the result of any testing by anandtech.
http://www.anandtech.com/show/4083/the-sandy-bridge-review-intel-core-i7-2600k-i5-2500k-core-i3-2100-tested/3
I also wonder about the source of the chart, and what the "1" note on the slide means.I suspect it means exotic cooling since 5.7 is an unusually high overclock.
You can overclock the graphics part, but that is largely useless on a gaming rig with a discrete graphics card.
Some usere have reported that their motherboard allowed a 2600 to specify multipliers greater than stock without error.
But... the multiplier was not actually used.
Being open minded, I would like to see some non photoshopped reports of high multipliers for non "K" chips.
Perhaps it would be best to open a new thread on this.