Ok, first, those articles are about statically compiled C/C++ and in particular targeting game systems that only support those types of applications. Runtime platforms like Java and .NET can do things do optimizations that those cannot like optimistically inlining methods where there is not enough information to prove that they never need dynamic dispatch, and making memory management for short live objects almost free by putting them on special parts of the heap.
But to take a step back - yes, it's usually possible to rewrite your app to sacrifice some readability to make it faster. I mentioned in my OP that I could rewrite my Swift app to use more structs and get rid of the reference types and it would probably meet my perf requirements... but it would be at least somewhat uglier and harder to maintain. Harder to maintain = bugs and lack of insight that might allow you to make bigger breakthroughs in perf later.
I'm not quite sure what the argument here is - We all want to use the most appropriate tool for the job. My original question was about whether these Clang changes might help improve Swift performance, which I believe is currently (ok, let's not say "crippled") hampered by excessive ARC calls.