I did a shootout between MX-2, MX-4 and MX-5. Results are very interesting.
This is no ordinary test because I used each paste on the die AND the IHS of a delidded processor. That means that the heat needs to transfer through two applications of the same paste and it really helps to highlight the tiny differences in performance.
Testbench is a delidded 120W TDP Xeon X5460 at 3.6GHz 1.35V with a 6 heatpipe Snowman 92mm dual tower cooler. Fan speeds are at maximum (molex powered). I used HWInfo64 to log data across two-minute runs of the Furmark CPU burner with 1 minute breaks in between. Ambients changed only slightly in the process but I recorded it during each run.
MX-2 Run 1 (24.3C Ambient) = 70C, 64C, 71C, 70C
MX-2 Run 2 (24.3C Ambient) = 70C, 64C, 70C, 69C
MX-2 Run 3 (24.3C Ambient) = 70C, 64C, 70C, 69C
MX-4 Run 1 (24.5C Ambient) = 73C, 67C, 73C, 72C
MX-4 Run 2 (24.5C Ambient) = 72C, 67C, 73C, 72C
MX-4 Run 3 (24.5C Ambient) = 73C, 67C, 73C, 72C
MX-5 Run 1 (24.7C Ambient) = 71C, 66C, 73C, 70C
MX-5 Run 2 (24.7C Ambient) = 70C, 64C, 72C, 69C
MX-5 Run 3 (24.8C Ambient) = 70C, 65C, 72C, 69C
I'll keep conclusions to a minimum but there definitely is a performance improvement with MX-5. Impressive numbers from MX-2 as well.
I also thought that all three pastes were very easy to spread. Wouldn't hesitate using all of them. MX-5 is very light blue in color and is hard to see on paper towel, but that's only a minor downside.