It depends on how much water goes through your overflow. If you overflow only drains say 800 gph, then you only need a return pump that will return 800 gph or a little less after head loss. If your pump is too powerfull, in theory it can drain the sump faster than the overflow can drain water back to the sump. If your sump wasn't designed right, you could end up overflowing your display tank, running your sump dry and burning out the return pump. Now, if the previous owner had the sump in the basement, I could see needing a Mag 24 to push water that high, but if the sump is in the cabinet under the tank, a Mag 7 or 9 would be plenty.
I have a 75gal AGA and I think the overflow is rated for somewhere around 600 gph. I have a Mag 7 that after head loss will push about 450 to 500 gph back into the tank, so I don't have to worry about my overflow not being able to keep up with the pump.
If you're looking for more water movement in your display tank, look into installing a
closed loop system, which a Mag 12 or higher would be pretty good for an 80 gallon tank.
