I have the THX II System in my 2018 MKC and think it is well worth the additional $995. I say that as someone who also thought it was well worth spending $7,000 on an "entry level audiophile" home stereo system.*
THE UPSIDES: As others have said, what you get for your extra money is "accuracy." For example, while you will hear the notes of a string bass or a guitar on the standard "Premium" system, with the THX system you will also hear the "thwack" of the bassists finger when he strikes a string and the "squeak" when a guitarist slides his fingers along the strings when changing from one chord to another. If you are into vocal music, you will hear the singer sing the notes and taking a breath. And I can't count how many times I've listened to a familiar song and heard an instrument in the background for the first time that I never knew was there. The HD radio sounds terrific when the signal is strong. And the DSP Surround Sound mode works particularly well with singer-songwriter performances, jazz, and the like.
THE DOWNSIDES: More accuracy exposes flaws in the source material. For that reason, low quality MP3 and some SiriusXM channels with low bit rate and high compression probably sound better on the standard "Premium" system. And, as others have pointed out, there is no subwoofer -- if you like rap or hip hop and you want a lot of bass "sound pressure" to rattle your windows and make the body panels buzz, the THX system definitely won't do it for you.