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.
