FWIW, I have a 2009 mini running a 40" Sony Bravia using mini displayport --> HDMI and it's working great, no picturing clipping or anything.
Also, I haven't tried Plex because I've got everything working how it want it as-is (eyeTV, front row). But I think my setup is not a conventional HTPC in that I eliminated the 3rd party DVR (just use eyeTV) and also use the mini as fully-functional mac (logitech dinovo edge mac edition is the perfect control solution). So YMMV.
Also, I haven't tried Plex because I've got everything working how it want it as-is (eyeTV, front row). But I think my setup is not a conventional HTPC in that I eliminated the 3rd party DVR (just use eyeTV) and also use the mini as fully-functional mac (logitech dinovo edge mac edition is the perfect control solution). So YMMV.