The idea of an exhaust system is to keep it flowing the same diameter after the collector all throughout to the tail pipe. GMPP is a good one for quietness and decent performance. Borla patented the fiberglass idea first in the muffler before cherry bomb and other competitors. Borla makes one for our cobalts too, but its the same as the GMPP, since borla helped GM on that one for catback.
All catback exhaust systems for the n/a have the same 2.25" diameter, so most of us go with E-bay and just replace the muffler. I have not yet, but plan on putting a Rock-it muffler to keep the same design. Just a bit more tone to it.
For supercharged and turbo applications, 3" turbo back would be the best idea "if" you are planning on adding more boost than what OEM calls for. Otherwise, you will have too much flow and can lose power.
2009 Cobalt LS XFE U74
Mods: K&N Typhoon Intake, Option Cat-Back Exhaust, Cosmo Short shifter, Maxxim Ahead 17X7's, MPx shorty antenna, FE5 Strut/shocks, Tein S Techs, Moog endlinks, Moog spring seaters, ZZP shorty header, LSJ downpipe, LNF front grille