We saw significant performance gains when moving from GCC (6) to clang (6). I don't think it'd be particularly hard to switch back at this point; this article provides some solid data for doing so.
Given that same level of effort (c.f. the article we're discussing) it seems like you could have done as well or better by moving to a more recent gcc instead. Or better, by working with both at coming up with a portable way to get LTO working.
I'm not really concerned with what you use to build (I mean, you have to pick some compiler at the end of the day), just with what seems to be "needlessly tight coupling" between clang/llvm and Firefox in a way that hurts the interoperable toolchain ecosystem.
Regardless, that feature being enabled when you're using suitable versions of clang/llvm/rustc doesn't preclude using LTO with other compilers.