X5s are not the fastest. There's Porshe Cayenne Turbo. Now that little son of a...

As far as performance - don't count on that muffler doing much at all, or any muffer in this case. Yes, the backpressure is reduced a little, but there's still piping before the cat that's pretty restrictive and the intake has restrictions as well. Even if you were to do the whole exhaust/intake setup, it wouldn't give you anything appreciable. Not too much at all.