Maybe. My guess is when they overclock they leave turbo mode on, so you could get a 10% overclock and turbo mode will still stack on top of that.
In most cases that's cool, but if you overclock too high and don't turn that off, the multiplier goes up by 1 in turbo mode. So you could have your system overclocked to 4ghz which may be at your limit, then all of a sudden your turbo mode kicks in and you start running at 4.2ghz and all hell breaks loose.
But yeah, I would save yourself the money and OC it yourself.
Also it should be noted that the new i7's don't use FSB anymore so when overclocking you adjust the BCLK (base clock). Doing so affects the QPI and memory controller, which for high overclocks means voltage adjustments. But you can overclock the i7 920 by ~30% and never touch voltage.
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