With commodore catbacks, all premade 2.5" ones are pretty much the same, in that they use straight through mufflers, choice seems to come down to what tone you want.
Redback seems to be the common choice, depending on who you talk to, Lukey is either slightly deeper or slightly raspy, best bet is to ask around for audio clips, or even ask people to take for a spin in their car with an aftermarket exhaust.
If you will be going custom made, you have the pick of a 2.25" stright through system to maintain low end torque and driveability, with marginal sacrifice, if any in the top end (an exhaust fitter I spoke too reckons a 2.25" striaght through on a V6 commodore will easily outflow what the motor can put out in terms of airflow) or a 2.5" to go a bit louder, and possibly have a bit of top end, also is a good choice if you plan to cam the motor later on.
The bare minimum I would recommend is a centre straight through offset muffler, and a hotdog at the rear to minimise drone, you could even run a straight through muffler at the back too to reduce drone further and make the exhaust quiet until you sink the boot into it. Pick mandrel bends if you want better flow, or press bends to minimise cost.