The current Motorcyclist contains a ride review of the 1600GTL and in a nutshell... they freakin' loved it, virtually no negatives to be found. They even praised the neutral handling to the point of comparing it to sportbikes. You don't see reviews that uniformly positive (in the opinions of all three reviewers, all of which are performance-oriented riders) very often. If the 1600 can resist self-destructing after x miles it sounds like it may be a new class leader, similar to the 1000RR.
But yes, the outright performance stats put it behind the 1300 in sheer acceleration, and maybe the 1200 as well. What was commented on was the arm-straightening torque, anytime, anywhere, and how effortless it was to ride, even for all its size and weight (and it is heavy.) That apparently was what BMW was aiming at and it seems they hit the target dead center.
But in the end the question (as always, but often overlooked) isn't whether the new bike is a little better/faster/smoother than the old (they usually are), but whether whatever difference exists is worth a $10,000+ price premium over a lightly-used slant 4, a very good bike in itself. That factor is always in the eye of the beholder.