The Megaflow overflows have a 1" drain and a 3/4" return... if you need more flow to the sump, you can always use both holes as drains, and put a return over the top.
Btw, I'm setting up a 75g AGA with a megaflow, and an oceansmotions 4-way, and have drilled the back of the tank for a closed loop, I'm not using the overflow for that... that's something you may also want to consider doing.
Nuno