I guess I wasn't clear. The k series processors have an unlocked multiplier. The i5 2400 does not. "Turbo" isn't really overclocking in the traditional sense, as it is built into the chip. And +4 bins is a 4% overclock. People buy the k series so they can get more than 100-200 MHz more frequency.
it's 500MHz (4+1x from turbo) + the extra for turbo on single/dual thread + whatever you can get with BCLK.
"While a combination of a 38x CPU multiplier and 105MHz Base Clock should have given us a 3.99GHz CPU frequency, the i5-2400 would only hit this frequency if a single-threaded application was locked to a single core (we set one instance of the smallfft test to run on core 0, for example). For the rest of the time, the i5-2400 fluctuated between a frequency of 3.78GHz and 3.99GHz – we’ll therefore say that we overclocked the i5-2400 to up to 3.99GHz. See How to Overclock an LGA1155 CPU for more details on how we overclocked all the CPUs on test."
http://www.bit-tech.net/hardware/cpus/2011/01/03/intel-sandy-bridge-review/14
stock clock is 3.1GHz (3.2 with turbo on), so this is not a 200MHz OC.
the good thing is, SB at 3.7GHz is still comfortable in terms of heat and power usage I think, and it's really fast.