Here is the result of my testing. Your firmware works. However, I am getting lower hashrates compared to current firmware with a dev fee! I am downvolting the S9 for power saving.
On the current firmware I am using I have stable 9,4Th with the 720Watts drawn on a meter. Settings 445Mhz/8V Dev fee is 1,8% so I am getting effectively 9,2Th for me. This means 12,77 Gh/W efficiency.
I tested you firmware.It is fairly stable (one to two-hours each settings) at the following hashrates. Always 8V:
445Mhz 8.5Th with 686W, efficiency 12.3Gh/W
425Mhz 8.0Th with 644W, efficiency 12.42 Gh/W
375Mhz 7.7Th with 611W, efficiency 12.6Gh/W
350Mhz 7,5Th with 588W, efficiency 12.75Gh/W
Only at 350Mhz I am getting roughly the same efficiency, but I then I get 7.5Th, not 9.2Th... They did something in the custom dev-fee firmware that allows higher hashrates.
Nevertheless, your firmware is genuine
Keep up the good work. I wonder how it would be with the newest bitmain firmware, but it is not possible to downwolt it without recompiling the "kernel module" C++ code - I am rookie so I do not remember the proper names.