[Chris Sanderson] “Being that the downmix is only for monitoring purposes, this setup would work fine for laying off 5.1 to D5 correct?”
No. Changing levels affects the SDI output. Map for six or 8 channels output, make all channels stereo and patch 1&2 to 1&2 output, 3&4 to 3&4 output and 5&6 to 5&6 output. Set all channels to 0 and pan hard left right.
Now you are set to output the channels at unity and discreet. The FCP default is not correct for output to tape.