I contact Stephan Tanner and ask him what size he recommends for a 750 gallon.
Since the sponges are inside the tank, in constant contact with the water, and trying to replicate nature, going as fast as you can through the sponges usually isn't the best idea. The organisms need time to build their colony, etc. so the right flow is what is needed. 10x is def not needed, it will only clog the sponges in a short period of time, it will be as if you had a canister on the tank instead of sponges.
If you get a big enough UPS that will probably work but for another alternative, you can get battery operated airpods made my penplax. I have mine running now, they are electric and battery, when the lights go out the batteries kick on and the filters keep on filtering.