Yes, I think the main advantage to your idea is flexibility rather than extra performance, as you gain more from increasing flow through the blocks than through the rads.
Re pumps, I have successfully "converted" a 180w draining pump (83lpm/5.5m) into a 3-stage unit capable of approx 45lpm/11m. For watercooling, but not PC
Its dead now, but I still have it lying around somewhere...