Is the GPH rating of a filter determined with the filter empy of media, with new media, or once established with debris and bacteria?

I would assume the GPH would be greatly different if run empty as to run with filter media that has been in there for awhile. The reason I ask is that my 30 gallon has a penguin 350 and an aquaclear 50 on it for a combined GPH of 550 or a 18.3333 tank turn over rate. I know that is more than enough filtration but could it be less due to used cartriges?

Filter manufactures definitely don't establish the rated GPH of their filters using dirty filter media. Whether or not they fill their filters with media before testing is hard to say... Some probably do... Others probably don't. One thing is you can count on. When the media in a filter gets dirty, the GPH decreases.