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.
http://mozakai.blogspot.com/2011/11/code-size-when-compiling...
http://mozakai.blogspot.com/2011/11/code-size-when-compiling...
> 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.