Just fyi, I have a 280GTS, 2xTX240. With one 18W DDC, I have 0.7GPM at max. To balance the noise I go with two 18w DDC.
Now I have about 1.2GPM at max, 0.45GPM at min, 0.7GPM at quiet level.
Remove some 90 degree fittings will help your flow but with such a complicated loop, it might be too hard...