This is not a fundamental limitation of asm.js. For example, there is already an extension introduced for compiled code that can be optionally used for improved performance: Math.imul. Similar extensions can be introduced later.
With the trade off that the code size is greatly increased and that the performance when falling back to pure JavaScript will be dog slow. If the performance of an application is only acceptable on the subset of browsers that implement asm.js then the portability advantage over PNaCL isn't worth that much.
The only actual numbers I've seen, from emscripten author Alon Zakai, show gzipped minified emscripten JS output as comparable in size to gzipped native code:
> If the performance of an application is only acceptable on the subset of browsers that implement asm.js then the portability advantage over PNaCL isn't worth that much.
The Unreal Engine demo has proven that, even for games, these limitations don't result in "dog slow" performance on browsers that don't support asm.js.
