Hi David,
I will have a lot more information in upcoming posts but I’ve been busy with work. In the meantime, I don’t think any discussion of multi-rotor frames would be complete without looking at Robert Lefebvre’s paper on the subject:
Given this information, it would probably be more efficient for you to build a Y6. The frame would be lighter and Arducopter in a Pixhawk or Cube would handle it just fine.
Robert Lefebvre @Rob_Lefebvre used to have a video on Youtube where he talked about all of these things but it seems to have been taken down.