The flow should be okay. As long as it is over 20l/h its should be allright.

Which pump? and rest of the Setup?

in the german forum we had a 60 pages long diskussion about flow restriction with the result that an eheim 1046 (wich is pretty much identical with the aquastream in matters of performance) is pretty much sufficient for any kind of cooling setup.the topic was opened before the xt series coolers were avidable but the setups used for the benchmarks were much more complex.some of them included cpu, chipset,graphicsaccellerator, hard disks, power supply and even hard disks.so due to the complexity of those setups you should be more than fine.if for any reasons the aquastrem would not suffise it would in my opinion result (at least if you test your setup with stadard clock speeds) in bad temperatures which shouldn´t be dangerous for your hardware.

so in the worst case scenario you could always sell the aquastrem on ebay or something but you would not damage your hardware and still bea able to replace it with a stronger pump.

edit:

but just to be sure you should tell us your exact cooling setup.

using the twinplex xts as gpu coolers should be possible.according to ac they have the same mounting system as the regular twinplex coolers but its pretty strange that one can not order them with a gpu mounting kit.i think its just because a normal twinplex is pretty much sufficient for every graphics card.depending on how long you want to keep your 6600gt you should buy a cooler for it or save your money for a cooling sollution for your next graphics card because it would probably also need coolers for the vram.

if you want a twinplex xt anyway for optical reason you should ask the ac support if you could order a twinplex xt with a suitable mounting kit.

even if the waterflow is reduced significantly the aquastream should suffise.