Sir, you have presented a question where what you will receive will be ambiguous answers at best. I have followed this mystery for a couple years and would conclude, when lowered - results may vary. There are many variables here. First, any CV's, in any car, will wear from use and mileage. Our XI's don't necessarily have a weak component but like others, should the boot rip or clamps loosen, the lubricating fluid seeps or actually slings out causing the joint to run dry and fail. Boots being thin material that flex and stretch have a limited lifetime. Keeping an eye on boot condition is recommended for all CV cars.
When the car is lowered, the drive position of the axle changes from a slight downward angle to horizontal or slight upward position. Some will contend this is the CV killer as not the factory engineered "normal" angle. I would suggest this is questionable as when the angle changes, it still remains within the normal operating range of motion. Think of driving over bumps where the axles are pushed upward into the "lowered" position and then return to the normal drive angle. CV's don't operate constantly in one position, they are designed to work in a range of axle motion. If not, we should avoid hitting bumps.
Lowering creates concerns such as wheel camber and and proper alignment that must be observed. Additional stress from stiff springs, struts, shocks and low profile tires should be considered. Strut towers and rear shock mounts need to be modified to handle the additional applied forces. The CV's however, are not affected from stiff suspension forces.
Personal observation - I think owners lower their car and simply miss the axle/boot condition. Their tired CV fails 500 miles later and the myth was born. Mushrooming the strut tower, tearing the rear shock mount and spine injury may be an affect but not CV failure. Too many owners have success lowering these cars where they payed attention to drive and suspension components involved. They report long term mileage without incident.
I believe when some install "sport" suspensions and big-ol', wide tires, they drive them hard for the pay-off. That's dandy 'cause now they gots 'em a race car but hammering the original axles ripping around corners and dropping the clutch may play into CV failure? BMW targeted this market offering the M3. Making one out of your XI is subjective and as stated in the opening paragraph - results may vary.
