Such a small space would definitely increase the flow velocity, but it sounds like your pump might not be able to handle the increased pressure drop. Your flow rate might drop way low, and although the block might still perform well, your rad performance will suffer.
This is one big disadvantage of Cathar's block: yes, it has a high pressure drop, but it's justified by the good performance even at low flow rates... but what about the radiator performance? It will no doubt suffer from the decreased flow.
Those engraved blocks all look like they would have a high pressure drop. Would work well with my beastly max 28ft head pump.
