I arrived at that by testing with and without the filter. Perhaps my conclusions are flawed, perhaps it is different using a D300, and I may be mistaken because I'm actually using a CC40M filter
Of course the CC40M has a stronger effect. However, it is still far from a full stop. It is difficult to determine the effect on the green channel because of the clipping, but it is clear, that the red and blue channels have lost about 1/3 stop. The gain in DR is the difference between the green and the red/blue reduction.
I have the Tiffan CC30M; its effect on the green channel is somewhat less than a full stop, while the red and blue suffer about half stop. The net result is rather less than half stop.
I don't have the transmission curve of the Tiffany filter, but I attach the graphs from B+W (they don't have a 30M). It shows, how much part of the blue and red spectrum as well is filtered. Add to this the fact, that the pixels are not dedicated to red, green and blue; "dominated" is closer to the truth. In other words, the combined effect is a mix.