Good Question,

my wife did similar. traded in her X5D for the new X4 w/M pkg and they removed the Nitto INVO i just put on that morning

i think the 275s the X5 Ran up front will be a great Rear for the X4 but the 315s are too wide for the X4 wheels and wheel well.

only one way to truly find out..... one of us will have to spend the time and money to mount them and see