Actually, there is wide variance in channels and showing as well.
Although HBO and Cinemax might have the same rate for each title each showing, it varies widely per title - so some titles can be at 8Mbps and others at 14Mbps - for example Star Wars 3 came in at 9.15Mbps.
To further complicate things, different systems limit the feeds in different ways. For example, you will almost never see the same same rate on same Titles on HBO on E* as it is apparently stat-muxed with other channels.
And then again, HBO on E* via 110W is different than HBO on E* via 148W.
Now, with Showtime-HD, the East Feed Distribution on 135W is higher about 2Mbps higher than the West Feed on 127W. And then again, on E*, for example, the 110W feed is about 2Mbps lower than the 148W feed.
Then you take Canada. Movie Cental HD has the widest swing of HD bitrates I have ever seen - a full 10Mbps. I have seen a title at 4.5Mbps (as unbelievable as it seems), Deadwood is distributed at under 6Mbps - yet newer releases touch 12-14 Mbps - and others run the scale.
But, if you take MC from BEV, its converted to 720p and runs a pretty constant 11.65Mbps. If you look at MC from Shaw Cable, its converted with bad headers for the resolution (saying 1440x1080i though its incorrect) and averages about 11Mbps - 12Mbps, though most titles are not that high in the original distrubtion.
You getting a headache yet?
So you can see, there is no simple answer. Believe me - I've tried for 2 years - and every time I think I can quantify something, you find another exception.
So the short answer is except for channels such as HDNET that run their bitrate at a constant 17.57Mbps (but then again, how do they do that with different bitrate masters - which you have to answer) there is not consistent number to really use.
All you can do is broad scale averages - and use that compare to what you know is going on with the original distribution via C-Band.