I used an xp2 on a 30g for a while. I think you are running about the best media for that filter, but I was shocked at how little dirt it cleaned out of the tank.
I was running an Rena Filstar xp2 AND a Magnum 350 on a 30g tank (I know, over kill, but that's the way I roll) and noticed that the Magnum would be clogged with filth after a week, and the xp2 could literally go a month with cleaning and still be less dirty than the Magnum.
I wish Magnums were as easy to service as the Filstars are, as disconnect and cleaning were a breeze. Magnums are tempermental, a chore to assemble and and awkwardly engineered, but polish water like no other filter I've used......