We just normalized HPHPc performance to make the graph easier to read. HPHPc actually got considerably faster over the same period as well; since both systems share a runtime, many changes helped both.
HPHPc probably got about 20% faster over 2012, even though nobody was actively working on it, mostly through happy side effects of work that was directed at HHVM.
As to whether it would have been possible to get better performance on HPHPc, I'll let people with more knowledge answer.
The benefit of having a single runtime for development and production (and production-like staging environments) is huge, though. There may also be benefits in the push process, which while fast when compared to most other companies, which will continue to deliver benefits for increased speed.