A thought experiment => Worst case scenario = condition is same as having flat plate (assume non-functional micro channels)Something to look at is the apogee drive ii. It is a reverse flow pump block. Though it has a pin based cold plate design vs the normal fins.
I am not sure if it should really cause any real difference though. You will have the same amount of water flowing through the coldplate either direction. And most are designed around low restriction. I am probably missing something.
It could be water coming together vs splitting at the coldplate causes more turbulence and more resistance?
Lehman
Read that the jet plate is to force the water travel through the micro channels. Water may just laze above the fins under low pressure. But then one can still consider that as a flat plate, and how bad can that be?
I think as long as there is enough head pressure from the pump and decent flow rate, then it doesn't really matter which way the flow is.