I too, am sad to see the NA V8 is gone and the name will be changed to M4.
As for your speculations, they seem very realistic
Knowing BMW M Division, they will probably tune the M4 to near 50/50 weight distribution, have one of the best suspensions setup and steering feedback. Performance wise, I would say the M4 may not be in the same league as GT3 & GT-R, yet. But who knows what tricks the boys in Munich have in their bags.
In addition, BMW would like the M4 to be more attainable by enthusiasts thus keeping the price at or below $80k. Can they tune the car to 550+ hp, 500+ torque as a "Stock" M4? Absolutely, but that will raise the cost tremendously.