[azakai]

I think there are 2 points - the one you said, to enable optimizations, and the other, to guide people to writing code that is easy to optimize.

I guess the question is, how much better can you do, over easy-to-optimize code. Or in other words, once something is valid in Strong Mode, how much do extra Strong Mode optimizations get you.

Such code should already be quite fast in current JS engines. The big, big difference is between the engine getting hard-to-optimize vs easy-to-optimize code. So I would bet that the bigger deal is the devtool aspect here. But, these are empirical questions.