Hi. Agreed on either vacuum pump or brake booster. I just went though something similar. For me the key test was that I could still stop the X just fine but I had to press very hard on the pedal at engine idling. In fact it was same as if I had the engine off.
Not sure if common issue on the 4.4 but on the 4.8s (in X and 545 etc) non return valve going bad and oil going into the brake booster is common.
Would suggest you try to rule the vacuum boost side of brakes in or out. You could have something else going but here are my details
http://www.xoutpost.com/bmw-sav-foru...cuum-pump.html