Try this mod, it does sound good. If you want louder go with after market.
http://986forum.com/forums/diy-project-guides/19770-diy-exhaust-mod-%96-gain-10-hp.html
or drill completely through the muffler like I did lol. 
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2003 Boxster S
| 987 Air Box | K&N Air Filter | 76mm Intake Pipe| 996 76mm TB | 997 Distribution T | Secondary Cat Delete Pipes | Borla Muffler | NHP 200 Cell Exhaust Headers |
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