I'm a little confused about some data about Lithuim ion batteries. For example, when reading this, it mentions that it's rated for 12.6aH but it's rated for 2C.

Does this means that you can get 25.2a (2*12.6) out of this battery? Would there be a sigificant voltage drop if you're drawing that much current? Also, does that mean that if you're drawing 25.2a continuously then you're battery life will be cut in half (assuming 100% efficiency)?

 

Basically, yes.
Internal resistance at .014 ohms will be the limiting factor.
25.2A x .014 = .3528 volts of internal loss during a 2C discharge rate.