Alex, it depends on the options you pass to the RewriteRule command. Using the [P] option will tell Apache to act as a proxy which means it takes the initial URL browser, re-writes it based on your rule and sends the request to the new URL itself, then waits for the response and returns it to the initial client.

If you provide the [R] option to RewriteRule it will do as you describe in your last comment, send a redirect back to the browser after which the browser will send another request to the server using the new port. In this case [R] is better than [P] because the Apache process is freed up straight away. However it also means that for every image you will get 2 HTTP requests from the browser to the server, which could be costly in very high traffic situations.

The better solution would be to use URLs that point directly to the Boa server for all your images to be served by Boa, this results in just one request per image and Apache will never get invoked.

Good work though and your right, the only way to know for sure is the benchmark.