There appears to be no gain in cooling performance with a DeepCool AIO over the stock cooler. Why is this happening?
Different coolers will have different cooling capacity. There are some higher end air coolers on the market that can outperform AIOs, but an AIO can usually handle major fluctuations in temperature a lot better due to the thermal properties of the coolant and how the heat is moved away from the source (to the radiator from the CPU) for cooling. There are a lot of factors that would have to remain constant, including the variability between units and the same exact situations, to do an accurate comparison between the coolers.