EDIT: Well, since only few actions actually make efficient use of the extra logical cores in an i7, you'd really only need a small overclock to get better performance overall.
I'd say 4.3 GHz on a 6600K would be fairly close. At that point, the i5 will beat the i7 easily in all applications using less than 4 cores whereas the i7 still wins in embarrassingly parallel workloads.
How come the i7 running at 4,0Ghz would beat an i5 at 4,3Ghz in 4 core processes?