First off, nice build, very clean. Well done.
Your water is getting to 6-7C above ambient after 1hour of load, which is decent. You wont be able to knock too much off that with extra rads, so I would stick with what you have for now.
It might be worth trying to re-seat your CPU block. You can change your thermal paste if you want, but you will not get much of a difference, maybe 1-1.5C if you go for some Shin-Estsu (see Vapor's test
for a quick comparison). You may get a lot more of a difference from a better mount though. Have a look at the spread of the TIM when you take the block off and see if you need more or less.
Other than that do as Charlie suggests and make sure that you rad is bled. You can do this by tilting and shaking your case with the pump running (jump it as you did for the leak test to remove the chance of buggering your HDDs or anything else). Make sure that your res top is on tight
, we don't want any little accidents...
Another method is to remove the rad from the case (with the tubes still attached) and shake / tilt it with the pump running. It looks like you have enough slack in the tubing to do this if you are not happy about shaking your case.
To be honest though 65C is good for a 4GHz OC with an ambient of 30C, I was surprised when you said that your H50 managed better than that.