With media and head loss you can pretty much count on hitting half of whatever is on the box.
I do think that the force ot the incoming siphoning water helps force water upwards through the return pump - hence the fact that a running canister filter operates under negative pressure, a vacuum.
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Thanks for the info, I think you guys are right about the siphon helping to power it. From further research, it sounds like the actual GPH with media and so forth is 225 GPH and the 350 GPH is just the motor itself.