First up strip everything out that you don't need. All the extra seats, the audio system, any electric motor that drives something not important. Then replace all the body panels with carbonfibre panels and all the glass with motor-sport grade perspex or polycarbonate.

Seriously, there's only so much you can do.

The X weighs on the dark side of 2000kg and that's that. You can change suspension components or the wheel/tyre package to achieve flatter cornering and sharper turn-in but these changes can have a negative impact on the ride quality and comfort, and can even have a negative impact on handling on poor-quality roads. The factory Adaptive Drive is a good bit of gear but I assume it can't be retro fitted.
Either way the X5 is about as good as it gets in terms of SUV/4WD handling.