Now, my findings have been that the D851 kernel source has had all of its comments stripped, making it a pain to merge stuff in to. The D850 has the comments in, as does the D855 source. So, I don't know what's up with that. I do know however that between the kernels, LG removed support for a panel or two between the D850, 851, and 855. Couldn't even get the damn thing (any kernel source) to compile until I did this:
https://github.com/ConnorBaker/lge_kernel_d851/commit/121e0c10062184bc21dd8b772ea057a200b94cd2
I'd use the D850 source as base for the T-Mobile variant, since the T-Mobile variant (via my research on the webz) tends to be the closest to the international version. Also, keep in mind that the defconfigs, although they share the same names through the different kernel sources, are different: i.e., the D850 T-Mobile defconfig won't match up with the D851 or D855 one's. Furthermore, there are defconfigs for Broadcom compatibility, which leads me to believe that LG is using both Qualcomm and Broadcom chips for Wi-Fi.
To my knowledge, no one has tried to build and boot the D850 kernel on the D851. So, only time will tell if the kernel sources, are in fact, interchangeable.
Warm regards,
Connor Baker