Did you ever think perhaps your custom water loop is putting a lot more contact pressure onto the CPU IHS thus forcing the gap between the IHS and the actual CPU die itself to shrink making thermal contact better between the IHS and CPU die since they use paste between the IHS and CPU die?. Considering the AIO system you where using has a plastic housing and mounting system that does not put a lot of pressure on the IHS?
The Silverstome HE-02 has a metal pump housing and looks like it can be tightened up a lot better then all the other AIO systems I have seen. It is definitely more like a custom water cooling loop mounting system. If anyone has yet to see Silverstones AIO he-02 240mm system and its he-03 120mm system I suggest you all take a look at it. It has a nice mounting system, all metal housing coated in nickle so it is nice and shiney, the radiator is a custom design that has a special patented fin design that increases contact area plus it comes with nice cover that goes around all 4 sides of the radiator that is white and makes it look great. Oh if you like lights the silverstone logo on the metal pump head lights up blue but it is not super bright from what i have seen.
Trust me If i had the money I would have picked one up in a second for my loved 2600k that is using a 120mm AIO now for almost 3 years and i could use the bigger radiator that I think would lower my overclocked temps 15-20c over the h50 type I am using now. Now and always I have kept my CPU temps under 70c fully loaded.
I am not saying tighten your water blocks onto your CPUs as tight as you can and crush the things, thats where common sense comes in...But i know My AIO cooler mounting system has the ring design like most out there and they leaves much to be desired when it comes to contact pressure.