corsa catback is one of the quieter catbacks. Most intakes are good, I prefer an over the radiator (OTR) like vararram. Then a good plug and play tune like cortex and you'll have a lightly modded G8 that isn't over the top, but more awake than what it is from factory.
If it helps, those above are the only mods I've done for performance on my gt.
2009.5 G8GT Premium OnyxRed Interior
corsa catback, vararam intake, cortex tune
custom audio, sharksfin, HIDs, LED Fogs, 20" MRR GT1's, Kayhan headunit