Flow is determined in part by the narrowest part of the system. This is called a bottleneck. If you introduce a bottleneck you reduce flow thoughout the system.
I understand what you're thinking but unless you have a truely massive pump, I think the loss in total flow will more then balance out the gains from the increased pressure at the block.
Widening though is much more common. Heatercores use very large diameters (5/8 ro more then 3/4) inside to slow the flow through them.
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