As the SLC AMG will be a higher priced sports car designed to compete with the Porsche 991 it will likely get more power and upgraded engine internals. That is simply speculation based on AMG precedent with the previous generation M156 and M159 motors. The SLK55 AMG will likely also adopt the M177 V8 although at a later point. It is unconfirmed if the name will change to SLK63 AMG although that is likely.
It is also possible that the base SLC and C63 AMG will share the M177 V8 while the higher performance version set to go into the 911 Turbo rivaling SLC AMG S will be the M178. M177 V8 output is targeted in the 450-500 horsepower range. With the performance package option raising output of the upcoming BMW M4 to the 451 horsepower range Mercedes may raise the standard output to compete with the heavier C63 AMG as they have a 50+ horse window to play with.
The M178 is rumored to be testing in trim up to 590 horsepower. It is likely that a variety of different tunes are being tested as Mercedes continues to benchmark the car against rivals such as the Nissan GTR.
BenzBoost has confirmed that the W205 C63 AMG will get a Performance Package software option. Mercedes already offered a P31 Performance Package on the W204 generation C63 AMG and they offer an AMG Performance Package for the M157 Twin Turbo V8 motors in 63 AMG vehicles that raises output.
How much will raise output over the stock 450-500 horsepower? That likely depends on the competition. The German turbo performance engine era allows manufacturers to simply play with software to leap frog each other and Mercedes has more room to do so by virtue of their displacement advantage over BMW and Audi as well.
Notice the software leak below also shows the chassis code as C205. Usually a "C" instead of "W" denotes a coupe. The W205 C63 AMG has been spotted testing as a sedan so this is certainly interesting. Interesting as well is the C400 model which will use a M276DELA30 bi-turbo 3.0 liter V6 with 4Matic all wheel drive. The Mercedes answer to the 335 and S4? Sure seems like it.
The next generation M3/M4 versus C63 AMG battle is going to be interesting.