Unfortunately Rhino is at least an order of magnitude slower than all the modern JavaScript engines (SquirrelFish Extreme, V8, TraceMonkey). I agree the JVM integration is very useful, I just wish there were a way to get the best of both worlds.
Port https://wiki.mozilla.org/JSctypes to v8, get C libraries for, well, cheap if not actually "free"... probably wouldn't take that much effort, but I'm just guessing. For bonus points, start sharing JSctypes-based wrappers between the two implementations.
Actually, if you're serious about JS-on-the-server, that's probably just about the most useful single thing you could possibly do to help it. Well, unless you're really attached to the JVM...
