Just re-tested my setup, and it definitely still buffers.
If I play a 1080p film (m4v format) there is only minimal CPU usage. If I turn on subtitles (tx3g) there is a slight increase in CPU usage. However if I use an .srt subtitle file my CPU goes to almost 100% on the NAS.
If I use my iMac as a transcoder, there are no issues.
In my setup, if I am going to watch a 1080p film with subtitles, I tend to use my iMac Plex Server. The only problem is my iMac needs to be running. This is usually not an issue, however my iMac sometimes goes to sleep midstream despite the settings to keep the iMac awake if there is network activity (this was changed in Mavericks as part of the energy saving changes) which means the steam dies as well.
I was thinking the Mac Mini would be always on and would be a good Plex Server for the ATV and other Plex clients.
Sorry if I have de-railed this thread on the ATV. It's really more a Plex topic than an ATV one.