"There are several writeups of large backends ported from node/python/ruby to Go which resulted in dramatic speedups, including drop in P99 and P99.9 latencies by 10x"

But that's not comparing apples to apples. When you get a dramatic speedup, you will also see big drops in the P99 and P99.9 latencies because what stressed out the scripting language is a yawn to a compiled language. Just going from stressed->yawning will do wonders for all your latencies, tail latencies included.

That doesn't say anything about what will happen when the load increases enough to start stressing the compiled language.