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.

I agree V8 does an awesome job buts it compilation phase is not done separate before running. Lot could done to gain the speed in those areas.

there is always something more that can be done