We're told that Canon will claim 15+ stops of dynamic range for the new Canon EOS-1D X Mark II. This claim was also made for the Cinema EOS C300 Mark II, which may be true at the hardware level, but in practice it may not actually perform to that specification. Let's hope it's actually the case with the new 22mp sensor.
Specifications for this camera have been extremely slow to come in from known sources. There is definitely a much tighter ship being run at Canon, but we do expect more to leak out as we approach an announcement in the next 4-8 weeks.