There's very little information about overclocking Xeons.
I suspect the information on your websites was taken from very few E5-2643-3 parts, maybe a couple guys posting their RealBench results or whatever. (I couldn't find a lot about
E5-2643-3 overclocking, but there are lots of hits about comparing E5-2643-2 vs i7-4960X overclocks.) Xeon procs aren't readily available through consumer channels, they cost far more than insanely overpriced i7-X enthusiast procs, and they aren't really designed to support overclocking - they can indeed be overclocked, but the people who buy them tend to be disinterested in gambling with the processor stability and reliability and longevity they've paid a premium price to attain.
Overclocking capabilities depend greatly on the motherboard, and I couldn't find any motherboards specified. This is only a 6C/12T part, but I seriously doubt all six cores could overclock from 3.4GHz to 3.8GHz without exotic cooling - the vast majority of Intel processors have only 2 or maybe 3 strong cores which can sustain full Turbo (and overclock) speeds.
You would probably do better with an i7-5930K anyhow, and you could buy three of them (with three chances at a mighty overclock) for the same price as one E5-2643-3.
"All opinions are not equal. Some are a very great deal more robust, sophisticated and well supported in logic and argument than others." - Douglas Adams
[/Korth]