This is a topic of constant debate with basically no end. There's a heap of information over on street commodores. Removing the rear resonator and replacing the standard muffler with a high flow one would give some small gains, but probably wouldn't be brilliant. My opinion is that the most cost effective option for a stock/very mild 5 litre is a 2.5 inch catback with one high flow muffler and maybe a resonator depending on what kind of note you're after. For more money, twin cats into twin 2.25 inch with balance pipe and one muffler per pipe. Although I reckon the single 2.5 inch would be the go and use the money you save to go towards decent extractors.
There are other options which may result in the loss of some low end torque on a stock engine but leave some "growing rooom" for further modifications (cam, custom tune etc.). There is the possibility of a three inch catback which apparently gives AWESOME noise and pretty good topend but is a bit sluggish at the bottom end. Another fairly popular system is twin cats into twin 2.5 inch and then back into single 3 inch over the diff with varying arrangements of mufflers. Once again anecdotal evidence seems to suggest a loss of bottom end torque.
I'm actually looking at getting an exhaust for mine, i'm probably gonna just go for a single 2.5 inch, cheap and boring (saving for extractors).
