One thing on chillers, as I understand it, is that you don't want to blow water through them too quickly. Per the other posts, there is an optimal flowrate through the chiller. I'd recommend a larger-than-necessary pump, then throttle it back with a gate valve (NOT a ball valve, which is more fickle to adjust). Like other's have suggested, I'd take the water out of the sump, and put it back there, too (not in the same spot). If it has some time to "mix", that is optimal.