The most important factor is water velocity through the water block. That is why turbulence is important. The faster the water flows over the water block's internal surfaces, the more cooling effect you get. That's why tubulence is important. Turbulent water has more velocity over the metal's surface than laminar flow does at the same flow rate (gallons/hour). So, what I like to do is have everything in your system 1/2 inch, except use a water block DESIGNED for 3/8 inch (like the Gemini low flow). In the old days, they used to do strip mining with water cannons. Water cannons were simply gravity fed, with the water going through progressively smaller pipes, then through a nozzle. This would produce very high water velocities that they would spray against a hill side, and this would wash away the soil very rapidly. Same principle here. Everything 1/2 inch, then step it down to 3/8 through the water block. This produces faster water flow through the water block. It's like putting your thumb over the end of the garden hose to get a higher velocity stream of water.