Mostly depends on the offset of your wheels, do you know it? I know my car is not an XI, but on my coupe I run 18×8.5 +42mm offset and I can fit 255/35/18 tires with my super low suspension.
Check out the tire/wheel/suspension database at Bimmerfest, see what other Xi owners are running.
Tire Rack and other retailers will sell short the possibilities. Every time I order 245/40/17 tires for my K1’s they call me to tell me they won’t fit.