I've spent most my adult life in Asia and have lived in several countries and cities that claim to be very serious about becoming the next tech hub. None of them have very liberal immigration policies. All have visas for highly skilled workers, all have visas for entrepreneurs and all have visas for investors. I have yet to see a single case where a founder could get a visa without either significant revenues or significantly more funding than YC originally gave. It's also pretty hard for mid-stage startups to hire foreign workers, even in places with exceptional government-driven startup initiatives such as Singapore.
In my perception, Silicon Valley's most critical weakness is housing prices and its most likely true competitor is Beijing—not due to openness of immigration by any means, but due to a larger base of technically educated citizens and access to what is becoming the largest market in the world.