The 20% comes from personal experience, maybe I should have said 'close' but that can also mean anything. Basically this is an area where the hotspot heavy duty compiler (C2) does the same optimizations as gcc or llvm. I would say that I would expect gcc & llvm to be within 20% of each other. The 20% is basically different tradeoffs made in the optimization pipeline (different peephole opts, different auto-vectorisation, register allocation algorithm, ...).

Java starts to seriously lags behind C++ when you need good control over the memory layout of your data. In java you can't have an array of objects, you get an array with references to objects. There are no elegant ways to solve that problem, the language just lacks the proper tools to do it elegantly. Memory layout is of course vital for cache efficiency, so that can give C & C++ a big advantage over java. Since this benchmarks uses arrays of doubles, this isn't an issue.