That's great for the Java code being JITed, but does nothing for the JIT compiler itself, or the garbage collectors, or any other runtime components that are written in C or C++ and compiled (and optimized) ahead of time.
Yeah, I suspect a good chunk of the performance improvement is related to GC performance improvement. The netty benchmark with pooled buffers compared to unpooled buffers hints at that (although I don't know for sure).