Personally, I don't think the Stromung is loud at all. It barely whimpers imo. But since you do have it coupled with the Borla header, that's probably your problem as to why its louder.
Any other type of muffler will still probably be loud due to the header, but you can try the Greddy Evo, or the Apex'i WS...but I'd just stick with what you have now. Or maybe an Apex'i N1 but stick in a silencer. Ooh I know, Apex'i (and others that copied it) has a cable operated flange in the exhaust stream that acts as a silencer. Full open you get regular flow, close it off a bit and its quieter.