FWIW, I did this locally (also happy to fork PB and check it in, certainly aids transparency). I've already updated the numbers in the benchmark doc but will continue to try squeeze more out of it.
As to 4x, my concurrency levels are significantly lower which could certainly explain it. Is PB juggling multiple connections increasing write lock congestion or are you serializing access, e.g. via a worker thread? Also happy to chat more (feel free to send me an email), I certainly want PB to have the best possible representation and maybe we can even speed things up on both sides