All right, here's a sample of what I would consider a low-flow block. There are two pathways from inlet (right side) to outlet that are mirror images of one another. This keeps flow through each pathway balanced pretty well. The pathway dimensions restrict flow rate to a fairly low number while creating a lot of contact area between fluid and block. The metal between the pathways needs to be sized for the heat load. If there's too little metal left, only the bottom of the channel benefits. If there's too much metal left, convection suffers and becomes the bottleneck (requiring more flow to compensate).
Herein lies another rub. Balancing of resistances requires specification of both flow rate and heat dissipation. You can only optimize for a given set of conditions. Yeah, performance would be OK across some range, but only truly peaks at one particular set.
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