If you look at Hiphop vs. HHVM there is certainly a significant performance upside in taking advantage of clangs or gccs ability to produce efficient code.
I'm not sure why Facebook decided to stop maintaining hiphop. Perhaps manageability wasn't great.
Perhaps. JIT compiler has to care deeply for compilation time. For AOT compiler compilation time is a little bit less pressing problem which leaves some space for optimization passes that take too long for JIT.
This would be pretty useful. Something like hiphop which could transpile into C or C++ would mean we (IncludeOS) could build tooling to run a node application on directly in a vm without a underlying OS.