The increase of performance you've gained so far with Lighttpd is not phenomenal, but helps to increase the website performance and reduces the load on your server.

I don't understand why a proxy/proxyreverse for a given request would put any less load on apache than just serving the static content directly.

To my (admittedly uneducated) mind, it seems that this proxy solution would be at least as much work for apache, if not more.