Same here... brakes (front and rear) perform well, but feel is "soft".I have owned both a 2010 R1200 GSA and now ride a 2013 R1200 GSA (last air cooled). I have the exact same spongy feel. Braking with two fingers gets scary when the lever pulls in an hits my other fingers against the grip. I bleed brakes every year with regular viscosity DOT4 and a GS911 and software to modulate the ABS pumps. I attribute the sponginess to the longer fluid routing and ABS pump and linked front/rear but will try the DOT 4 low-viscosity next time. Will follow this thread. I can't believe that the master cylinders are bad on your practically new bikes and wanted to add that I have always had the same concern since the bikes were new. It is obvious for anyone who rides and grabs the brake lever even without actually riding it.
If you pull the lever twice sequentially it is much more solid. Seems like you have built up some pressure at that point? I've never had a problem coming to a stop and often ride pretty hard including off-road. Brake performance is good but brake feel is not good.