before you start laughing at what kind of muffler i or anyone else put on their car you might want to consider actually looking into the product. ill make it wasy for you.. ill even post the link for you since your are obviously not familiar with the search feature on most web browsers..here is the links for you..
http://www.tccamotorsport.com.au/sebring.htm
and wouldn't you know here is the second link..
http://www.sebringtuning.com/new/main.html
and ill just let you know incase you dont want to read the about us section. Sebring performance tuning was created by Jim Cook Racing that has dominated racing for 23 seasons with an astonishing 24 hour endurance record of 7 wins in 14 starts, three 24 Hours of Daytona 12 IMSA lap records, 5 Show Room Stock National Endurance Championships. The Jim Cook banner has flown over 35 different types of cars on almost every circuit in the United States, plus races in Canada, Mexico, Australia and Japan.
so before you go off and attempt to flame someone for what performance products they put on their car you might want to do the research so you dont look like the fool...
and now that thats done, to answer your questions o.d. is the term for outside diameter. and yes the main cat is the one after the downpipe.