No one knows exactly how much faster Haswell will be over Ivy Bridge. Some people say 10% - 20%. I think it will be less than 10% because I believe Intel is focusing on making Haswell more power efficient than more powerful. That's because they need to expand beyond PCs and laptops where growth is slowing down. Desktop PC sales are actually shrinking a bit. But in any case OC'ing will give you better than stock performance.
Intel needs to get CPUs into tablets which is a growth industry. But they need to use CPUs that uses less power than laptop CPUs. That's why I believe Haswell will offer a relatively performance increase. Broadwell is Haswell's successor and will be released in 2014 which I believe will increase performance by 10% - 15% over Haswell. 20% would probably be wishful thinking, but who knows? It might be possible.