I'm not not so sure about that. I reckon multimedia means it will play from Bluetooth and USB via iPod or USB flash drive or smartphone. You should see that the Nav option gives you the 7 inch screen or the upgrade to Premium Audio (which is a half price upgrade if Nav is already specified) gives the 7 inch screen. Other than that the 5 inch is standard with no DVD playback.
I checked my old price list (April 2013 for MY14). Bluetooth and USB connectivity come with all the systems, Multimedia or not. DVD comes only on systems marked Multimedia, so I guess this is basically synonymous.
You can however have High Performance with a CD (not multimedia) with both 5" and 7" screens. As long as it's a multimedia system (DVD), it comes only as 7". Hope this helps.
As for the Premium system, whenever I am alone in the car, I crank the volume up until I can't hear myself singing along :-) - the sound is fantastic, no distortion, and the volume bar is still only a notch over half way.
Oh, one more thing - I think premium system has Dolby Digital and ProLogic II surround processing, and the High Performance does not. DPL works on MP3 music from USB, and does a pretty neat job. You can adjust the level of surround effect and turn it off if you prefer music "as it is". I am not a big fan of artificial processing and effects, but this is really good, I keep it on most of the time.