Be careful when you are thinking about a 1200 gph return pump. First make sure your overflow will handle it. IMO your return pump may not need to be even that large unless you are trying to use it a main source of circulation in the tank which usually is not the most efficient way to go. A gen-x 6000 with a 3-4' head and not many elbows I am guessing will flow around 800gph. That is still quite a bit through a sump. Of course the pipe size, number of elbows, number of entrances, ect. will help determine better. Are you going to have any other powerheads in the tank for water movement?