Time to settle the score and bust some myths. I tested MX-4, MX-5, NT-H1, GD900 and KPX on my delidded overclocked Core i5 4670K running at 4.4GHz @1.35V with a Hyper 212X cooler and fans on 100%. I ran the Furmark CPU burner for 2 minute runs with 1 minute breaks in between to get repeatable temperature readings. Each TIM was applied on both the die and the IHS to magnify the slight differences in performance as much as possible. Everything was cleaned with IPA as best I could and I made it as level a playing field as humanly possible. Wall power draw for each run was hovering around 170 watts (I was using the iGPU).
I used HWInfo64 for data logging and these were my maximum temperatures for all four cores on each run:
Arctic MX-4 Run 1 (Ambient 23.1C) = 77C, 76C, 76C, 74C
Arctic MX-4 Run 2 (Ambient 23.2C) = 78C, 77C, 76C, 74C
Arctic MX-4 Run 3 (Ambient 23.2C) = 78C, 77C, 76C, 74C
Arctic MX-5 Run 1 (Ambient 23.6C) = 72C, 71C, 71C, 70C
Arctic MX-5 Run 2 (Ambient 23.2C) = 73C, 73C, 73C, 72C
Arctic MX-5 Run 3 (Ambient 23.3C) = 73C, 72C, 73C, 72C
Noctua NT-H1 Run 1 (Ambient 23.6C) = 70C, 70C, 69C, 68C
Noctua NT-H1 Run 2 (Ambient 23.2C) = 71C, 72C, 72C, 70C
Noctua NT-H1 Run 3 (Ambient 23.3C) = 71C, 72C, 72C, 70C
GD900 Run 1 (Ambient 23.2C) = 72C, 72C, 72C, 71C
GD900 Run 2 (Ambient 22.9C) = 75C, 74C, 76C, 73C
GD900 Run 3 (Ambient 23.3C) = 76C, 75C, 75C, 73C
Kingpin KPX Run 1 (Ambient 23.1C) = 71C, 71C, 72C, 70C
Kingpin KPX Run 2 (Ambient 23.3C) = 70C, 69C, 70C, 68C
Kingpin KPX Run 3 (Ambient 22.9C) = 70C, 70C, 71C, 70C
Keep in mind that the first run is essentially a warm-up. Run 2 and Run 3 results are more repeatable and more indicative of the actual performance.