Other than grossly poluting the environment for noise, not really.. I'm not a tree hugger or anything and my talon doens't run a cat because it's turbo, but on this car I don't bother cause if it's really not making a huge difference and the added polution is just not worth it.
How about something like this?
http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&item=8013418811&rd=1&sspagename=STRK%3AMEWA%3AIT&rd=1
Since it seems like you probablly can weld and fabricate, take some 1" pipe and run it from the entry before the muffler and out the tip by-passing the muffler totally. Apparently this sounds quite nice and add's a lot of grunt to the boxster exhaust. Chances are if you run a modded car at the track you can fab it up in your garage. I have a tig welder in my garage and will probablly whip this up in an afternoon.