25% on average.
More like 25% best case.
Going by Cinebench on this page the 6700K gets 22% more points per GHz than a 2700K.
The increases for each gen (2700K -> 3770K -> 4790K -> 6700K) were:
6.5%, 5.7%, 8.4%
For a 25% IPC improvement the Skylake needs to post scores at clock speeds 43% higher than SB. Can't find any benchmarks with that much of a difference.
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