I'm all for using even the detuned LS3 in the CTS as an upper-level non-V series engine. The 400+hp that the G8 GXP is pumping out is nothing to sneeze at, and the car weighs about the same as the CTS, if not a bit more. If absolute power is needed, then use the 430hp version. Either way, neither the 5-series, E-class, nor the S6 are pushing those kind of power numbers. Only 380hp's the highest I see a V8 pumping out from this segment (from Mercedes' 5.5L V8). So in either tune, the LS3 trumps the other motors, both in power, torque, and fuel efficiency. Refer to my posts about the '09 XLR for my take on using the LS3 in Caddies.
However, the 400hp 3.6L DI twin-turbo V6 found in the Velite concept would work out nicely too, if it's more fuel efficient than the LS3. Either way, when competing with the 5-series and E-class, an upper-level engine is needed. Either the LS3 or a turbocharged V6 pumpung out 400hp or close to it will work great.