The CTS-V shell is more stable with equal, maybe a bit more turn-in than the Mazda6 but less mid-late corner steering. Mazda has more on-power steering and is twitchier. I haven't tried it on foam tires with my own car but I drove my buddy's car with the CTS shell on foams and it felt pretty dialed
For foam tires though, The best shell I've used is the Parma Alfa. Car feels planted with plenty of steering.
Hope this helps