Let's face it though, the main string operation in web apps (or most software) is concatenation. I strongly doubt there is any point in converting back and fort

Let's face it though, the main string operation in web apps (or most software) is concatenation. I strongly doubt there is any point in converting back and forth between UTF-16 and UTF-8 just for that tiny advantage in addressing individual code points - and even then, not quite 100% of the time. For search-and/or-replace (or anything that can be done with regexes), I'm pretty sure that UTF-16 has no advantage over UTF-8, as you can make the state machine operate on the bytes directly.