For idle testing, we booted up the system, allowed the CPU to stabilized at ~0% usage, and then tested for at least 10 minutes. Then we used RealTemp GT to record the CPU temperatures once the temperatures were stable. Then, for load testing, we used Prime95 to load the processor to 100%, and allowing at least a half hour for the liquid temperature to stabilize. An hour "cool down" period was given after each Idle-Load test cycle. We tested two different conditions of the i5-2500k, the first is completely stock and default settings of 3.3GHz, and the second is overclocked to 3.8GHz with AUTO voltage. The single temperature recorded was the average between the four cores. Since this is an enthusiast/overclocking-oriented product, we also increased to 4.0 GHz to see how far up the temperature curve we'd gone.
The standard fan speed ramp we use has the Seidon 240M keeping our i5-2500K at very cool idle temps right around the 30°C mark in both cases, and it was nearly-silent. When ramped up to 100 percent (which is where the other coolers were tested) we see the Seidon besting everything we've tested on this test bed. But I'm sure this isn't the test you were looking for...
On the business side of things, we see the Seidon 240M does exceptionally-well, and stacks up comparably to a push/pull 120mm all-in-one watercooler, the Antec Kuhler 920. One key difference when comparing to the Antec Kuhler 920, however, is that the Seidon is substantially quieter. At full fan speed, the Kuhler 920 is, frankly, unusable. When the Seidon 240M is at full fan speed, it gets to a low hum, but nothing that would keep me from being able to live with it next to my workstation.
Better yet, when we go to 4.0GHz, the temperatures only increase an additional 2 degrees Celsius, and maintaining and average temperature in the low-50s at 4.0GHz is awesome. Overclocking boundaries, as you probably know, are reached so rapidly because of the exponential increase in heat output as you increase frequency. We are seeing a relatively shallow thermal saturation curve from the Seidon 240M, and this is with only two fans. In push / pull (which we'll test as soon as we get a matching pair of fans in the mail), you can expect things to look even rosier.
As far as noise-to-performance ratios go, the Seidon 240M is definitely up there with the likes of the monstrous NH-D14, without the strain on your motherboard and the overbearing aesthetics.