In regards to speculation, here's MT's excerpt referencing (predicting) a twin-turbo LT1 in a CTS-V and on a future ZR1... debatable as why not hope & hold out for this one too? Maybe even a vert version... for 2016, 2017, etc.
"Better yet, it’s slated to get a seven-speed manual gearbox option. Even better than that is Cadillac's plan for an ATS-V+, which will get an even more powerful V-8, and could arrive as soon as 2017. Could the V-8 be the new supercharged, "625+ hp" LT4 from the 2015 Corvette Z06? We sure hope so. What's not getting the LT4 is the new Cadillac CTS-V. Instead, it gets a new twin-turbo version of the Corvette’s LT1 V-8, an engine that's also slated for the next Corvette ZR1."
Source:
http://m.motortrend.com/wot/1401_cad..._turbo_v6.html
In any case or whatever GM'a plans are... what a great opportunity to have so many choices! A good problem IMO.