Yeah I just wondered if you watched it being done, that's a big area of copper to remove in one go, probably had cooling fluid running over it all the time. Must be so nice to have the right equipment
I suppose it depends on your complete system but as my coolant is cooled to the same temp all the time 14°C I have tried slowing the flow to a trickle, and only then does it make a difference to max temps. I just don't think huge flow rates will make a difference. what you need to do is test it out as it would be pointless to fit bigger barbs for no reason.
If you intend to use a air cooled rad and a smallish coolant content then having to greater flow rate could allow the coolant to pass through the rad without time for losing it's heat and then you could get a heat cycle.
Best to do a simple test. Get it up and running and take some accurate max temps cross referenced with case temp and ambient, then just crush the inlet tube a bit to reduce the flow. If you get no change in max temps your flow rate is adequate and increasing could make it cool worse.
Not sure what you mean by your soldered plates. My personal opinion is you want the water or coolant touching the core for best cooling, this is not really a viable option, so the next best thing, (other than silver), is the thinnest possible copper separating the water form the core, which is why I milled the base of my waterblock so it was less than 1mm thick. that gave 5°C to 6°C lower max temps, (it was 4.6mm thick). I have an old CPU here, (gig T-Bird), and I'll be trying out running the water actually on the core soon to see if it helps.