My 2013 Model S weighs on the high end of the 7 series. My understanding is that BMW used to recommend 35/38 for its 245/45 R19 tire size on the 7 series. The 7 is rear wheel drive with a 51/48 weight distribution (front/back). My 2013 Model S weight distribution is the reverse of the 7 at 49/51. Depending on flavor, I believe the 7 series weighs between 4,200 and 4,700 lbs.
I'm going to go ahead and try reducing the pressure, I just wanted to make sure there were going to be no dire side effects. Seeing what pressures BMW recommends is helpful, and Musk saying 45 PSI is for efficiency (not ride comfort) tells me there is some room to play.