That would most likely be limited to your radiator cap.
Actually the radiator cap controls the pressure on the radiator side of the thermostat. The RPM of the pump impeller dictates the pressure inside the block when thermostat is closed. By tapping into the block and the pump input one is diverting water from the block through the manifold creating a by-pass circuit around the thermostat that is always open. The pressure and the amount of flow through the bypass is a function of the tubing diameter. Which I think should be kept on the small side . . . I am using 3/8 hard lines.
I sell pumps for a living. Large municipal stuff. I deal with pumping systems FAR more complex than the cooling systems on a car.
You're absolutely right about using smaller lines to divert flow in the way we're discussing. If you're going off the T-stat housing then you should use lines as small as possible to transfer the heat while minimizing the reduction of flow through the engine where it's cooling.
Mine are large because they come off the heater core. I can afford to waste a large amount of flow with minimal loss to the car's heat.
The radiator cap has nothing to do with this stuff on this system because it's a vented system at the radiator.
The 3/8" line you're using would be what I would use.
