I read that there is a custom picture setting for the C300 that attempts to match it with the 5d mkII. Is it possible that that could somehow be used on the XF300?

Just a thought.

Sorry but no. The preset files for the C300 and XF series are indeed
similar in structure, but are also just slightly different, enough so that
they are incompatible with each other.

The C300 has a Gamma Table called EOS STD Check out my Gamma tests on Vimeo at Canon C 300 Gamma Curves & Moire test on Vimeo
This is EOS std gamma table mimic's the Canon EOS std Gamma found on the 5dmk2 but unfortunately you don't have such a gamma table on the xf300

Too bad, that would have been nice. Even without the shallow depth of field, or the low light sensitivity it would have been nice to help match them up a bit easier.

Thanks for the replies.