You would need to put a VERY large sac on top of the factory one in order to develop enough pressure to force water out of the bottom of the bag (where the drain pump is attached) and through the drain fitting in the side of the boat. So there's really no risk to doing that, it's mainly just the convenience issue of having one larger bag connected to your factory system for automated filling and draining vs. having to manually fill a bag after your factory system is done filling.
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