every cpu is different. the overclock you get on one isn't a guarentee of an overclock on another. generally Vishera chips should get to 4.5 without any issue at all... after that it comes down to your motherboard, psu, and how lucky you were with the cpu.
I've seen most people are able to get up to 4.8 if they work it... around 5.0 or 5.1 the vishera gets a bit unstable. i do know a guy who was able to get his up to 5.5 with an overclock for everyday use. But he had a very robust cooling system and motherboard, and he admitted it was oddly unstable from 5.2 to 5.4, he just kept pushing higher to see how high he could go. And fell into the strangely stable 5.5
The point of all of this is, to say "you can't tell till you unbox the chip and spend a few weeks tweeking it"
When it comes to overclocking CPU's, you get into something known colloquially as the silicon lottery. Based on where the chipped your CPU off of the wafer at, you get best/great/better/good CPU's for overclocking, or you can have one that might not even see 4.5 stable without a metric ton of voltage to keep it there. When you're tweaking the CPU's voltage, you find a stable point for your freq, and tone it down by 0.005 vCore at a time until it's unstable, won't boot, etc. then you go to where it booted, and prime95 it for 24 hrs. No errors or warnings = stable overclock. My i5-3570k sits happily at 4.2GHz/1.240vCore, and I'm not giving it 1.360vCore for 4.5GHz.