You also lose the abilty to control the On Board Computer (OBC) functions if you remove the factory radio unit.
Now if you really want to get serious about this, here is what you do. Someone sent me information a few months ago on a project he was working on to retain his factory radio, factory changer and Sirius while adding Navigation. To do this he moved some components around to make it all happen. First he moved the factory radio down to the ashtray. To do this he build a custom center console cover to accomodate this. He will be building an Alpine monitor into the old radio location. With this setup everything will still work, including all of the cluster information and stalk controls. As a final touch he is painting all of the wood in the X5 to a Piano Black to match the new console.
Check out this handy work:
