I think it would look nice as well. One of the X5's saving graces is the chrome tipped dual exhaust on the 4.0 model. Looking under the rear end there is plenty of space for a second muffler although the space doesn't seem as wide as the stock one. It may require swapping the stock one out so two like ones can be put in. As for peformance advantage, I don't believe there would be any since it most likely won't be a true dual exhaust. I don't think the underside would accomodate it as well as the availability of aftermarket headers for the PILOT. But, it sure would look neat ... The MDX has a chrome tipped exhaust that looks nice, just imagine 4 tips sticking out the back instead of the 2. Perhaps a Photoshop pro could work some magic and show what it could look like. Just thinking of things to do once the engine has broken in and the warranty's life begins to end.
Generally to see increased performance from an exhaust upgrade you need to do quite a bit.
Very few normally aspirated engines will benefit from an exhaust upgrade unless other steps are taken to increase airflow & fuel delivery. (turbo/supercharged engines are realtively easy to modify and the benefits of uprgardes exhousts are easier to exploit)
Even in cases where you are upgrading from a single catalytic converter to duals, the total exhaust volume is usually adequately handled by the stock system.
There are exceptions to this, but very, very few, as modern engine design looks to optimize efficiency at every step, whereas older engine designs many have made sacrifices to keep noise levels lower.
The simple reason for the popularity of exhaust/tailpipe modiifaction is they are highly visible. In contrast, things like changing the air box, intake manifolds, cylindedr heads, pistons, fuel injectors, are pretty mcuh invisble!