Tim,
I have a 2004 XK8 (4.2 liter) which is stock except for a K&N filtering system and minor exhaust modifications. I notice that yours is the supercharged model, and is probably the 4.0 liter. My stock system included a single large muffler/silencer located approx. under the front seats followed by twin exhaust pipes going into a set of intermediate silencers followed by pipes going over the axle into two final silencers and ending in a set of small end tips. After some experimenting, I removed everything after the single large main muffler and installed twin pipes going back and over the axle (the stock pipes flatten as they pass over the axle and choke the exhaust). My new set of pipes, fashioned by my local muffler shop, are mandrel bends going over the axle. Between the axle and the rear bumper, I installed the stock 7" silencers and ended with a larger set of end tips (3 1/4"). The sound works for me - a mild rumble at idle, stronger exhaust at hard acceleration, with no popping when backing off the gas, and a very mild exhaust note at expressway speed. As far as performance, I like to think there is enhanced performance through the mid-range. But then again, we all like to think that our modifications result in some kind of improved performance - maybe yes, maybe not. If you care to talk about this, give me a call at 504-390-1119.
Hope this helps,
Dan