I think you got them backwards. The XTS is the full sized all wheel drive sedan. The ATS is the Buick based ride.
You obviously haven't driven the Regal T-Type. Pretty solid platform and for a fuel efficient platform, can actually handle and perform rather well.
Bear in mind that the CTS-V Sedan is still the fastest production sedan in the world (At least around the 'ring). It matches the higher horsepower S8 0-60 and its suspension and brakes are right up there with the best of them being made by Brembo and all.
But you are right...they are still using pushrod engines. Sort of like Harley Davidson sticking with 1903 technology and adding fuel injection.
I too have had several Cadillacs as well as a Pontiac G8 GXP (LOVED this car but it wasn't worth it in the end) and 2 Pontiac Bonneville GXPs (Northstar based cars). None of them would even compete with the M5s, B7s, E64s, S8s of the world. But they have their advantages that make them a decent value for those who can't pay the premium for the best of the best.