This is why i went with flux less radiators the one i am using is Aquacomputer AMS 840 rad and AMS 480 rad and they cool really well. And of course no more pesky crap, flux, gunk just a prestine clean radiator that doesnt pollute your precious wc system (but always do a flush before use). Sure it costs more then regular rads but you get what you pay for. Another contender is the Watercool MORA rads same principal round tubes no flux. I only use a single D5 pump and cool the cpu and gpu, no restriction issues even tho these rads are more restrictive then your average flat tube rads.
I was fed up with some of the regular rads i used in the past some worse then others.