Don't let me talk you out of your Superdart pumps. They will serve you well for a closed loop system. They just need large plumbing and short runs without nozzle restrictions.
Take an extra day or two to play around with the plumbing before you ad salt & rocks. You will quickly see how they perform with the Seaswirls. You will also see how much force you get out of one lone sump return, alternating sides. You have to strike a balance between flow that reaches the opposite end of the tank without blowing the paint off of the corals on the way there
Your sump return line will need some nozzles to diffuse the flow. I would aim for a maximum of 1,000 GPH out of any one return port, but that's just my personal preference.