Overclocks are generally dependent on the "CPU lottery" where some CPUs overclock better than others. In general, people have experienced lower overclocks with Haswell generation CPUS (i5-4670k) vs Ivy Bridge CPUs (i5-3570k). Both generations can typically be OC'ed to 4.2GHz (there have exceptions for both though). But it appears the percentage of Haswell CPU capable of being overclocked beyond 4.2GHz is lower than Ivy Bridge CPUs. That's not to say a i5-4670k cannot be overclocked to 4.7GHz. They can, but fewer of them can reach that clockspeed vs. the i5-3570k.
In term of average performance increase over the generations:
2nd gen Sandy Bridge > 1st gen by 12%
3rd gen Ivy Bridge > 2nd gen by 6%
4th gen Haswell > 3rd gen by 6%
So basically assuming 1st gen, 3rd gen & 4th gen Intel CPUs are clocked at the same speed; Ivy Bridge will provide about a 19% overall increase in performance, and Haswell will provide about a 26% overall increase in performance.
Lastly, no more socket 1155 core i5 CPUs will ever be manufactured. However, Intel intends to release the Broadwell generation (5th gen) socket 1150 CPUs (same as Haswell) about 1 year from now so, potentially there is an upgrade path. But Broadwell is more of a refinement of Haswell than anything else. The manufacturing process will shrink from 22nm to 14nm, power consumption will be reduced, and the iGPU performance has been speculated to increase by (30% - 40%). No word on CPU performance, so probably another small 6% increase in performance.
The expectations is that Skylake (2015; a totally new CPU architecture and socket) will return to double digit % increases. However, that likely depends on how close/distant AMD's CPU performance will be.