"More importantly, when this loop matters enough for me to care about a 5x speedup, I want the vectorization to be explicit and predictable. I don't want an unrelated code change or compiler update to quietly turn it back into a scalar loop."

Only tangentially related but this is by far the most painful part about optimizing code for JIT compilers like V8. Even changing a constant from 1 to 1.0 somewhere else can change the optimizations performed and lead to an unexpected performance decrease.