Ultimately yes, but if you are happy with your current code, you probably won’t see as much as a speed up as you did with polygons. The one caveat would be if you have a large amount of static billboards ( > 10000), then I would wait a release or two, since that’s one situation that Entity billboards still need to be optimized. Of course if you switch now, you’ll get those optimizations “for free” as soon as they are ready without any code changes on your end.
There’s also other reasons to use entities as well (camera helpers, infobox, easily aggregates visualization into a single object, etc…). For anyone writing new code, I would recommend using the Entity API as much as possible and only fall back to primitives if there’s something you can’t do with entities (and then tell us about it so we can add it to the Entity API as well)