XBMC is a wonderful thing. Actually, I believe a rPi was used in our apartment to set that up.
I've got Plex running on a Mac Mini. Problem is, when you're running both the head and the server on one device you lose efficiencies. ...and when your Mac Mini was new in 2007 you start tripping over your 1080P. an rPi XBMC head seems like a delightfully tinkerish method of resolving the problem.
Maybe you should consider upgrading your mac Mini to a later ~2010 model then! Performance is no issue on these machines because Plex' GPU hardware decoding implementation of the 320m is really, really good. You can play back a full bitrate 1080p Bluray rip with < 10% CPU usage and get plenty of headroom for other stuff in the background (multiple other PMS transcoding sessions, unpacking, etc). Add a small SSD and that HTPC/Homeserver-machine should last until 4k gains traction. The rPi is just too slow for high quality playback. OpenELEC comes pretty close and out-of-the-box HDMI-CEC is amazing, but the bad performance ultimately ruins the whole user experience for me.