I feel your pain, I also worked on a large MMO style mobile game backend, but my situation was a lot better because as you say AWS is a lot more mature at this point.
Dealing with limited resources such as MySQL and Redis is a problem you still have to fix when you move away from PHP though.
As for the Facebook runtime, for us it was a case of drop in HHVM, fix a handful of issues and get a 4x or 6x speed increase for free. You only need to change the way you write if you move to Hack, but I think I agree with you that's probably getting to the limit of the effort reward tradeoff before you rewrite in a different language.
Dealing with limited resources such as MySQL and Redis is a problem you still have to fix when you move away from PHP though.
As for the Facebook runtime, for us it was a case of drop in HHVM, fix a handful of issues and get a 4x or 6x speed increase for free. You only need to change the way you write if you move to Hack, but I think I agree with you that's probably getting to the limit of the effort reward tradeoff before you rewrite in a different language.