'opening' up teh sump pump will depend on other factors also. i.e. What size drain in the overflow, and what size pump, and what head loss to the return line nozzles inside the tank.
Typically you would not want to 'open' up the pump, but just achieve teh flow you want, not more.
If you are wanting more flow or circulation in your tank, you have other choices, powerheads, waveboxes, Oceans Motions, ...sqwd
Good luck
Bill