That is the old story, people building flat octos claim that they are more efficient (compared to X8), and people (like me) building X8 claim, that the loss in efficiency can be covered by weight reduction.
I can build a frame for a 5kg (AUW) X8 with 150gr (14" props) , I am afraid, no flat octo frame with 14" props less than 500-700 gr is possible (with comparable stiffness). If you cannot save weight at the wiring when switching from an flat to an X8, there is something wrong.
Here an X8 with 60 min flighttime:
....
This copter has 14" props, and the copter has only 1,35kg possibly somebody can share the weight of a flat octo with 14" props for comparison ...
best regards
Ferdinand
P.S. I already thought, that the times of 10-12min copter has long gone ...
Sorry if you've explained this already - what prop pitch do you use top and bottom?