Dynamic range is a very specific, quantifiable thing. The 5D mark ii has been tested properly, scientifically, by Robert Primes ASC and ProVideo Coalition and they've found between 10 and 11.2 stops of dynamic range, the discrepancy probably due to the 10 stops being determined before the flatter picture profiles were around. To suggest it has 8-9 at the most and that 11 is an urban myth and to back this up with feelings and subjective non scientific opinions is not helpful in my opinion. Or you are using a different definition of dynamic range based on how much you like the look of the noise and how the codec falls apart, which should be called something other than "dynamic range". The FS100 is rated by Abel Cine at 11.5 stops and the non SLOG F3 at 12, if you can find anyone out there with both who says the FS100/F3 has at least 3.5/4 stops more dynamic range than a 5D, I will eat my hat, that is a HUGE amount of light and is completely bonkers. 1 stop is a lot of light and stops aren't a linear scale.
Back to the C300 vs 5D3, it's too early to tell on lots of things but I imagine they would intercut well as Noel mentioned, similar DR, high iso and canon colours will help here, assuming that downres looks good on the 5D3, if not the resolution difference might be jarring as the C300 has a full 1080 lines of vertical resolution so looks nice and sharp, I don't think it would intercut well with 5D2 footage.
Back to the C300 vs 5D3, it's too early to tell on lots of things but I imagine they would intercut well as Noel mentioned, similar DR, high iso and canon colours will help here, assuming that downres looks good on the 5D3, if not the resolution difference might be jarring as the C300 has a full 1080 lines of vertical resolution so looks nice and sharp, I don't think it would intercut well with 5D2 footage.