If you know the machine and platform ahead of time, not really. For frontend JS this isn't the case. But for backend code it absolutely is the case.

Sure, theoretically the JIT can sit in the background, see which functions are called the most and how they're call and then re-JIT pieces of code. In practice, I'm not sure how often this is done and if you even gain much performance. You MIGHT in a dynamically typed lang like JS because you can find out a bunch of info at runtime. In something like C# though? You already know a bunch at compile-time.