Transcoding performance on 10.6 vs. newer versions of OS X

I'm running Plex media server on an early 2008 macbook pro 15" with a 2.4GHz Core 2 CPU. I'm running 10.6 Snow Leopard and its just barely fast enough for on the fly 1080p transcodes with most files. Its stumbles on some files, and is most of the time too slow to burn in subtitles to the transcode.
My question is if anyone thinks I would get better performance on a newer version of OS X? Or perhaps even running windows?
Thanks