Although they may be able to reach high overclocks (and are great processors), clock speed doesn't mean much.
Unless you're comparing two processors in the same lineup/generation, directly comparing clockspeed is a waste of time and is irrelevant.
Please keep in mind that this isn't a quantitative measurement, it is based around mathematics and assuming that the processors performance is linear.
A SB i5 @ 4.5GHz is equal to a IB i5@ 4GHz (since they perform around 12% better per clock iirc).
So, a IB i5 only needs to be at 4.5GHz to be equal to a SB i5 @ 5GHz.
Now, if we go further on, Haswell performs another 6% better than IB. So an IB processor only has to be @ 4.2GHz to be equal to a SB @ 5GHz.
To conclude, you cannot directly compare processor clock speed between generations.