Aside from having too much flow for your fish, you can't "over-filter" in the sense that the nitrifying bacteria will only grow relative to the amount of ammonia produced by your fish, not the amount of media you provide. Flow rates aside, you will eventually get to the point where all you are doing is wasting money for no improvement in water quality. I'm not familiar with the filter you have, and have no idea about the stock or feeding of your tank, so can't comment on whether another filter will be a good investment in your case.