lol i think the forum likes it when you mention corsa, because they are a forums venderHad my 05 V8 fitted with a 4in. Corsa system today. Very good mod. Nice appearance 4in. Duals, and it sounds great. Just like advertised, quiet when you want, and gets mean when you need it. Mr. Browning hooked me up at the Corsa Plant in Berea. It's a pleasure doing business with him, as he is a "certified" motor head like most of us. What a fantastic facility also, 50,000sq. ft. with expansion capabilities in the near future. Do yourself a favor and look into these Corsa exhaust systems, it makes driving the STS way more fun!
I'm in no way affiliated or do I work for Corsa, they just have a great product.
On autoanything.com, they have a Borla cat back listed for the Cadillac STS. Strangely it is listed only under 2006, 2007, 2008 models, but I don't see any reason it would not work on the 2005.I got the corsa exhaust and love it but they do not have a cat-back as far as I know. Borla has one that is discontinued for the STS, but I prefer the corsa anyways, the pipes going back are already big enough on the STS.