The way I understand it is that high flow rate through a low resistance waterblock is better.
While small channels will cause an increased pressure drop across the block due to frictional and orifice restrictions, to overcome this a higher pressure is required. A bigger pump is usually the first step to overcome this and with that comes more heat dissipation to the fluid from the pump.
The aim is to optimise base thickness and have high flow with low resistance (so you can use a less powerful pump). That is not to say that you should run a bunch of 3/8" holes down the block to achieve high flow and low resistance - that would reduce the thermal transfer rate and hence the efficiency.
It's one big juggeling act to get high flow, for turbulence and low resistance for reduced pump sizing. Turbulence is necessary for increased thermal transfer of heat to the fluid and is achieved either by flow rate or by physical dissruption.
So, in a narrow bore tube, where the boundary layer maintains laminar flow to a greater proportional distance into the tube you would probably need to use a physical means to break the boundary layer and promote non-laminar flow, especially in a low flow block where laminar flow will be the predominant domain within the tube.
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