Also take into consideration that rather or not your overflows will handle it, do you want that much flow going through your sump? Sounds like you have a lot of flow in there already also, sounds like you have your corals etc. already in a whirlpool bath.
If you're point is to remove the powerheads you can put in a closed loop system and just use a J tube down into the tank about half way to use as the inlet if you can't drill it since it's already up.