It would be either the RS4 Avant (if they'll ever ship to the US) or the M5 for the near future (just signed up for number 4 for the M5 at my dealer).
However the real "beast" to get would be the new RS6 Avant ....
I've also been thinking on getting the Jeep GC SRT-8 later this year to save miles on my X but after looking at the JGC I am no longer sure as it still a Jeep with lots of plastic on the inside
...
As most of us have noticed getting out of the X5 into another SUV is almost in any case a step down...
Haven't decided if I am going to sell mine in October or keep it ....
- Jan