There is at least one language-enforced performance improvement: C++17 enforces some amount of RVO. Thus some stuff that would have been copied in previous stan

There is at least one language-enforced performance improvement: C++17 enforces some amount of RVO. Thus some stuff that would have been copied in previous standard is now not anymore. More standard types supporting move semantics & such also mean that generic code which correctly calls std::move / std::forward may do fewer copies the more recent standard you use.