THX issue - have you tried going into the Viera settings and changing one of the other options such as Game or Custom to be idential to THX Cinema? I have taken custom and set it first on THX settings then tweaked it a bit based on home needs and also other calibration listings by others elsewhere. It looks very close to THX but you could just put in the THX settings. I went to Netflix on the VT50, opened up a movie. A few moments in, I went to the menu and it shows it is set up on my "custom" setting.
If you go to the settings and look up the THX Cinema, you can go in and see what all the settings are. Simply copy them to custom or any other display setting that you normally would not use. Once done, set your system to use that particular custom settings. You should have THX Cinema settings working with custom. You may need to go in deeper for other advanced settings but start by giving this a try.
XBMC and Plex - I find it is good for movie playback more than music. I'll give a nod to iTunes and ATV for being more familiar to most Mac users. You can however set up XBMC and Plex to have other interfaces and apps incorporated to expand beyond a 'directory' like structure.
I would normally just copy the settings in Custom like you state, however with the THX there are settings in the hidden menus that are set by the factory that users cannot get to without a password. So the THX settings are calibrated internally to the set at the factory in order to he the THX certification. So just copying the THX settings into custom will not be the same.