the tubing down is meant for the "noise cancellation". like there is always an air in a loop. over time it's getting more. tubing down keeps the air trapped in the rad and it's not making noise in the pump.
thicker rads. the heat transfer works due to temperature difference. when air enters the rad, it starts to absorbs the heat. with each mm, the air becomes hotter and the temperature difference between rad and air become smaller. eventually this difference is so small, that the heat transfer is very ineffective. That's why high speed fans providing high airflow required to take advantage of the thicker rads. also, thicker rads, are usually have lower FPI (less fins) than thinner rads. so the total cooling surface is not that much bigger. that is basic thermodynamics. if you want further proof, look for radiators comparison (on the same site) with fans up to 1500RPM - tolerable noise.
what people strive to, is to have fans at lowest possible speed (preferably sub 1000RPM) to keep their systems next to silent. that best served by larger surface radiator. For your CPU, a single 240mm radiator (true copper one) is enough. 280/320 rad would be the "optimal" for great cooling at low noise. anything beyond that is kinda waste.