I owned a 2010 R1200RT, which was the same air cooled version of the boxer used in last year's GS, so I was curious about the new water cooled boxer. Although my RT moved along fairly well, and was commendably smooth for a boxer twin, it did leave me with a sense of being a bit mundane.
Today I rode the new water cooled boxer, and I was completely shocked by this new boxer. The new engine has incredible snap, and the front end of the GS would easily come up on roll on acceleration in 1-2 gears, and get light in 3rd gear as well...amazing considering how pedestrian the outgoing engine felt. The GS absolutely stomps now, and is a thrilling bike to ride just in terms of sheer engine performance. I have ridden previous gen GS's, and this new bike has a new found character.
The engine is noticeably more aggressive sounding, and smoother than the older boxer as well. This engine certainly feels like it was given substantially more than an extra 15hp, and I would say that in it's current form it will have plenty of torque and power for two up riding.
Although a GS would not be a bike for me, I think the upcoming new RT with this engine might be. A sub 600 lb sport-touring bike with light weight, agile handling, and finally an engine with the power to provide some excitement. We shall see what BMW comes up with for the RT, as no spy shots have even been revealed yet, but the new boxer engine is a winner.