As an extension what it does has to be specifically specified.
As an extension it can't be upgraded without making and proposing a new extension.
As an extension it passes all work to the browser vendors who each need to implement it
As a library it can be updated and extended whenever
As a library it only needs one implementation and everyone can use it
As a library it can do whatever it wants, no spec needed
From the discussion above it doesn't seems like it needs to be an extension. It doesn't seem like there is some specific OpenGL functionality that needs to be exposed to make it possible. It also doesn't sound like a speed issue given that the resulting shaders are up to 10x slower.
Also as a library it should be easy to patch it the same way the WebGL Inspector patches itself in or various other libraries that patch things like WebGLRenderingContext.prototype.compileShader