I'm wondering if footage shot in B&W on a 5d would suffer less compression issues than colour stuff (being that there is less information to try to squeeze into the available space). This is opposed to shooting colour and changing to mono in post. Or does the camera still compress three separate channels as B&W?

Good question.

One way to test it would to to shoot a test scene in color and then in B&W. Compare the datarate. If the B&W file is smaller for the same duration, then you're not gaining anything. If the file has a similar size, then you are.

The reason is that the camera would be putting zeros in for all the chroma. h.264 compresses zeros very efficiently, due to Hamming Coding and the lack of motion. If the data rate is still high, then those bits are likely used by the luma channel.

yup-

Colour = 4.8 mb/s
mono= 4.7 mb/s

That's nice to know.

B&W films are often done with color filters. As I recall, Hitchcock used red. You could use an optical filter. You can also apply a 2-d white balance offset in the menus toward red (great for people) or yellow (great for keeping the sky from blowing out in 8-bits). Of course, optical filters can be best, since they manage the light before it even hits the sensor.

Back on the compression thing, it would be cool to record a smoothly changing surface in color (and remove the color in post) and in B&W and see if there is a difference. The weakness of the Canon encoder is on nearly flat, smooth gradients. Nicely lit balloons would be a good test.