I changed the title of your thread. Many members have a "2002 SLS". (Seville Luxury Sedan)
The pipe diameters change all over the place - the roll bender used to form the pipes for the mass production jig necks parts of the muffler and Y pipes down to almost 1 5/8" in places. You would probabkly need 2" inlet/outlet mufflers and have a good exhaust shop weld up the necessary adapters.
Have you seen the CORSA cat-back for your car ??? www.corsaperformance.com, #14150. Not cheap, but well worth the fun and sound for you - plus the 10 - 15 hp way up at redline. +16 hp on my STS up at WOT 5600 rpm.
Thanks for the info, it's been snowing and raining here nonstop so I haven't really had time to check the setup to see if it's like center Inlet and dual outlets or center inlet and offset outlet I just have no idea