C&D quotes the curb weight for the CTS 2.0 at 3721 lbs. and the 3.6 at 3889.
You're conveniently leaving out the fact that the A6 is AWD. It matters, a lot, in every phase of the drive.
I don't get how you say "everyone says weight is one of its strengths." Sheer mass, in itself, is not a positive attribute. Nobody says otherwise. You look for structural rigidity, or you look for reasons to justify the weight - such as an AWD system or, in the case of a convertible, properly engineered bracing that serves to stiffen the chassis.
Like I said, the CTS isn't *bad* - in fact, it's quite good. but it's not best in class. The A6 is. In 2014's 4-way comparison test, the CTS finished second, although it got good commentary on being "sporty".
The 5 series was last, behind the E350. BMW is a shade of its former self.