If it's filling up that fast I would raise the cup as high as it will go, just be careful of the rubberband thing that holds the cup up, they are known to snap.
What manufacturers claim and the truth are sometimes two very different things. I think you should be fine for now, just see what it pulls when it breaks in and judge fromt here. You can also add a mag3 oump at anytime if the MJ1200 isnt enough.
Filter floss is like white polyester fluff, you can get it at the petstore or even in a craft store. Just make sure it has no flame retardant additives.
here;s some shots of what I use. The filter pad floss is on the left and the bag is the poly fill you can get at the petstore or craft store.