It should be fairly stable at speeds up to the 130mph speed limiter. Gusty crosswinds may affect high speed stability of any vehicle, and given the X5's fairly good aerodynamics and 5000lb mass, it's perfectly fine to go as fast as you wish.

*Always obey posted speed limits and regulations. Always wear a seat belt. It is the driver's sole responsibility to decide an appropriate vehicle speed depending on traffic and environmental conditions.