The way I'm visualizing the flow through the system as it is now doesn't seem to be the best it could be design wise. It looks like the hot water is rising off the waterblock, going up the tube to the rad to cool off, and falling back down the same tube like a heatpipe. There doesn't seem to be much incentive for the water to pass through the rad and back down the other tube creating the desired flow through the waterblock.
If a single pass rad was used that was tilted very slighly up at the hot side and the waterblock was put at a point not at the bottom of the loop but somewhere just above the bottom on the hot side of the loop I see a lot more of the desired flow going on.
Of course, your reality may vary.
