haha, i wouldn't try it. neither should you!

The pressure difference from a moving car is sometimes great enough to lift off sunroof glasses off their tracks and fly off the car, or completely shatter the glass. The new vette had issues with it's roof ripping off from the pressure differences caused by higher speed driving. I'm not saying the X5 cover will get sucked through, but it is smart to leave some sort of venting to help equalize pressure within the cabin.