Radeon™ RX 560 | High Performance Gaming Graphics Card | AMD
AMD, the makers of Rx560 state that this gpu reqiures a 450watt psu.
I agree that you can power a gpu with a weaker psu but the performance deficit will be visible.
Sorry but you don't understand PSUs.
They don't have a limit at what they are rated at. Even high-end PSUs don't have that limit at the rated wattage, they normally can go up to 50% more till the protection system kicks in and shuts it down. The rated wattage is what they are designed to provide the best current and voltage balance, closely conform to the efficiency standard they adhere to, lowest harmonics and noise etc. So even a small difference like 30W is actually nothing, he will not find performance degradation at all but the PSU with its low quality build, harmonics and noise will gut the GPU anyways.
AMD recommends a 450W PSU for long term use but essentially a system like his does not consume over 70-80W itself. Thus his system will at most consume about 160W, adding efficiency loss (let's assume a very high 30%), it would be 208W which is still less than 420W. Yes, with time and the temperature it usually works at, will make the PSU less efficient and lose capacity but that can't be quantified reliably.
Go read a comprehensive review of a PSU, there will be a lot of technical stuff but you will understand the operation. Specifically Over Current Protection (OCP) deals with the cut off.
