I have bought several Akropovic (Ti) exhausts for my bikes mainly for weight and balance. Doesn't make sense on a street car... taking out the spare gives you 3x better performance!
BTW You do realise there is next to no power difference between exhaust systems from the cat back?!
Anyone who tells you a special exhaust pipe after the O2 bung and catalytic convertor will gain you power is yanking your chain.
Most 2.5" performance pipes will flow 400+ CFM, doesn't matter what brand.
Your LS3 breathes in 305 CFM max (restricted by the manifold BTW), so no need to panic about power loss through the exhaust too much!
Your HSV stock exhaust restricts power mainly because of one small easily-fixed chink, a 1 1/2" NVH restrictor immediately after the header to cat flange:
That's less than 25% diameter of the combined port(s) size at the head. Simply replace this bit with metal hi-flow cats and 2.5" mandrel to header flange as secondary for $600-$700. This will definitely maximise performance even without a tune, make it rumble and bark louder and crackle clearer. Want it even LOUDer?! Swap axleback (rear cans).
Take a look at this video of two high performance G8s both flowing well over 330 CFM.
http://www.grrrr8.net/Forums/showthread.php?t=33743
The one on the left has a
stock catback (without above restriction).
Exhaust pipe performance catback is ignorant pipe-stroking about brands and bullshit. Forget catback power, which is a given thing when you replace the restriction as above.
After that, try different cans at the back because that is where most of the sound should come from. Ideally the exhaust will give you a low rumble at idle, no 116hz cabin drone at any time, and open up when on cam.
Consider a good value 2.5" from NAAF or even Xforce $750 job sounds ok IMO. You like it loud so turn it up an judge for yourself:
[video=youtube_share;kUuJi3O85hQ]http://youtu.be/kUuJi3O85hQ[/video]