I have the Solo axle back and AFM turned off, big improvement over stock. You could start with this and work your way forward if you want more; if you completely replace the stock system first and find you don't like it, you're kinda stuck. Opinions on this approach may vary. Another option is to try and find a G8 group in your area (if there is one) and listen to different exhaust setups in person.
'09 LR G8 GT Premium w/Sport Package, Roto-Fab w/HSRK, Cortex 91 tune, Solo Axlebacks, PPV struts/shocks, Russell SS brake lines, Breyton Race GTS rims, Michelin Pilot SS rubber