Klonk
If you increase velocity the pressure will decrease. The higher the velocity of the water the better the heat transfer coeficient will be but the increase in turbulance will result in a higher pressure loss through the block. So the balance would be between getting a good enough pressure from the block outlet so your radiator can work properly and getting the heat transfer in the waterblock. I dunno how to work this out, it'll probably take a couple of months with good cfd software.
The channel should probably be at least a couple of mm wider than the CPU die. Don't know for sure though it'll probably need some testing
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