It is hopefully apparent that the 40% increase (whether it was meant to indicate over leading Javascript engines at the time or over the ActionScript 2 engine) does not in any way indicate there is headroom for a 40% performance increase if static type performance optimizations were applied to a modern Javascript engine.
Many performance improvements come from being able to do code analysis during generation, which is already a trade-off for environments doing in-time interpretation, code generation and optimization.
Large performance increases would most come from removing dynamic language complexity, such as having use of the 'class' feature added in ECMAScript create non-mutable properties by default for function declarations.