The question is why overclock? if you are a gaming, for example, you might see ZERO differences. And at best 2-8fps. Not enough to justify overclocking. Toms has proven this over and over.
Most games are GPU dependent and relating those to CPU overclocking is totally irrelevant, but games that are CPU dependent like Flight Simulator 10 for example, show remarkable improvement when the CPU is overclocked, along with audio and video encoding apps as well, etc.
Now if you don't mind showing some references of the statement of "Toms has proven this over and over", I would like to see where you're getting that information.
As one that does overclock their CPU and have seen many performance gains from overclocking it, more than enough to validate doing it, I just cannot concur with that statement.
Many simply do not reach the performance gains they would like because they cannot keep their CPU cool enough to reach the higher levels, but for myself running a 5ghz overclock 24/7 on every CPU I have owned for the past 3 years, I totally disagree!
Bobshaw's original request is for an air cooler that will support his 4.5/4.6 Ghz overclock to stability, so obviously he knows what he wants and what he is asking for, he is not asking should he even overclock!
Since the Noctua NH-D15 will fit inside Bobshaw's BitFenix Shadow Tower case with it's 165mm height dimension requirements I have to recommend it over the Cryorig H5.
A terrific base for the modern PC, Shadow easily accommodates long graphics cards up to 320mm in length, and CPU tower coolers up to 165mm tall.
It is presently out of stock at the UK link Bobshaw provided, but IMO worth waiting on it getting back in stock, but that website is not his only UK purchasing outlet, I am sure it is available at another UK Etailer.