The relevant bit is around line 300 in thumb.php (after the comment "Stream the file if it exists already"). Also, it does return a 304 when appropriate (of course your browser needs to send an If-Modified-Since header for that to happen, which it probably won't do). thumb.php streams the file from a PHP process, while Special:Redirect just sends the browser to a new location which is served directly by the web server, so that's indeed less overhead, especially for large files.

Anyway, thumb.php is internals, while Special:Redirect is a public URL, so it is always more appropriate to use the latter (or the API).