As I understand it, potential performance gains are higher than 2x, and we're still in a pretty early stage. It depends on your application.
Also, as has been mentioned by others, WebAssembly could perhaps evolve to take a greater role, leading to a greater potential impact of performance gains.
Finally, if the complexity does in fact outweigh the benefit, the idea would eventually fade away. So far it looks promising. Time will tell.
These are already old numbers, and section 7.3 says 33.7% more efficient. Were one to run updated numbers, and use more than Chrome and Firefox, I'm pretty sure the numbers would be better still.
I note that the comment in 7.3 says the biggest speedup vs asm.js is in validation, so I assume this means the measurement includes startup. This advantage is easy to concede to WebAssembly due to its design.
FWIW I trust that the numbers were good when the Google and Mozilla folks gathered them, but I had nothing to do with numbers!
In particular, I haven't looked into validation. JSC doesn't treat asm.js any differently than JavaScript, so the comparison would be different as well.
Also, as has been mentioned by others, WebAssembly could perhaps evolve to take a greater role, leading to a greater potential impact of performance gains.
Finally, if the complexity does in fact outweigh the benefit, the idea would eventually fade away. So far it looks promising. Time will tell.