That muffler will only be worth the money if you like the way it will look hanging off the back of your car. Presently you have another muffler there, its sole job is to keep noise down. We affectionately call it a lunch box (it looks like one.) Little to no hp gain will be made by removing it. Doing a lunchbox delete will simply increase noise some and replacing that with that muffler you linked to above will mostly only alter the exhaust note.
The main restrictions are further up starting with the exhaust manifold/main catalytic converter (our "header",) flex pipe, secondary cat and overall diameter of the exhaust piping. Replacing those with better flowing parts will net you some hp gain and get you a better sound.