Sumps are optional...you can do fine without one. IMO
, you should try to keep the sump, especially if it is mostly there already. There's too many benefits to it.
You didn't mention what sort of system is returning water to the sump, but I'll assume it is some kind of overflow, gravity based system.
Since you can't control the maximum flow of the overflow, you have to slow down that sump pump. This can be done by putting a ball valve in the return line between the pump and the tank. You can just cut off the flow until the overflow is able to keep up.
You also didn't mention having any problems with the tank. If the sump pump was actually draining the sump faster than the overflow can suppy the sump...then your display tank is headed toward overflowing.