I guess that's why Kotlin became so popular in the Android world - all the new & shiny features are present but you still can target older bytecode versions
IMO the problem with calculating precise savings is there is no "standard" use-case that could be used as a reference. I took one real-life application as an example in order to be more precise than just "it's somewhat faster", but the benefits will differ based on many different things (platform, app size, frameworks, etc.).