What is a reasonable expectation?
The high end Core i7 5960X operates at a standard frequency of 3 GHz and has a maximum turbo of 3,5 GHz. The i7 5930K, the fastest 6 core processor, operates at 3,5 GHz and has a turbo of 3,7 GHz. If you chose to overclock any of these two we have found out there is a lot of headroom on top of these frequencies.
We have tested three 5960X CPUs ourselves and ASUS has shared their experiences with a lot more of these processors. According to ASUS - and our experience is similar - a Core i7 5960X combined with decent water cooling can het up to an average speed of 4,5 GHz running on 1,3V Vcore. Should you only achieve 4,4 GHz this means you have a slightly poorer example, and getting up to 4.6 or even 4.7 GHz on 1.3V means you have got an exceptional chip. As the 5930K and 5820K feature six cores in stead of eight usually speeds of around 4.5 to 4.9GHz are achievable.
Please note that the aforementioned speeds have been achieved for benchmarking purposes, running on 100% CPU usage for only a few minutes. When you are looking for a stable and safe 24/7 overclock and keeping the temperatures within safe limits you should aim for around 4.0 GHz on the 5960X and 4,2 to 4,3 GHz on the 5930K and 5820K. These are a significant increase on the standard speeds, meaning that you can get a lot more performance out of your Haswell-E regardless.
To give an idea of the speed increase when overclocking the Core i7 5960X on standard speed reaches a score of 14.3 in the Cinebench 11.5 benchmark, when overclocked to 4,5 GHz we reached a score of 19,3 points: a 35% increase.
What do you need?
This does not come as a surprise for most readers, but in thi sworkshop we have to emphasize the importance on good cooling when overclocking the Haswell-E successfully. If you want to achieve results as we described in the introduction the minimum you will need is one of the best air coolers available (like the Noctua NH-D15) but we would advise the use of a water cooling set up with a minimum of a 240mm radiator. When using a standard air cooling setup you will reach the maximum temperature threshold very quickly, mainly due to the amount of cores that the haswell-E processors have.
Also very important and often overlooked, the PSU used when overclocking. We obviously do not think that a system with a Haswell-E processor will have a cheap no name PSU, but when operating at 4.5GHz the 5960X processor will draw around 25A from your 12V rail. When using a multirail PSU you will have to ensure that a single rail can take a minimum of 30A, something that most branded PSUs of 650-700 Watt are able to supply.