I haven't had the opportunity to hear the base Model X sound system, but I can say that the Ultra High Fidelity Sound System is reasonably good but not spectacular. However, I'm very glad I didn't stay with the presumably inferior base system, since it must be rather bland if the Ultra High system is considered premium.
If you're expecting something along the lines of the spectacular Mark Levinson system found in the Lexus LS460, it ain't even close. (And this is coming from a professional musician who would know the difference.) But imagine the alternative if you don't upgrade, so I'd recommend spending the bucks... but limit your expectations. It's a solid system with good fidelity, but the low end could be tighter and more pronounced, and the soundscape could be much wider. It sounds like it was built by very competent engineers using instruments to measure everything, rather than having an aural expert use his ears to craft a work of art.
I can't edit my original post since the edit window timed out, so instead of revising my prior post, I will post amended comments here.
First off, IGNORE what I wrote above! Silly me, I forgot an axiom of testing sound systems: the audio source makes all the difference. When testing out the sound system the first time, I relied on radio, Slacker, Bluetooth streaming from an iPhone, and my friend's USB stick. All sound was reproduced well, but it wasn't anything special.
Fast forward one day, and I returned with my own professionally-mastered USB stick (just a fancy way of saying I converted my lossless iTunes tracks to VBR Unconstrained AAC on a USB 3.0 thumb drive), with hand-selected tracks designed to put the Model X Ultra High Fidelity Sound System through its paces. What a difference!!
While I still don't believe the Model X sound system rises to the level of the Lexus Mark Levinson, I take back my ignorant and uninformed statement that it wasn't close. The Model X ROCKS when the right source material is played. The only track that seemed to overtax the subwoofers was - funny enough - 1984 by Van Halen. (I've seen that opening track from the 1984 album literally destroy subwoofers before, so this isn't anything for Tesla to be ashamed of. The good news is that it
almost pulled off reproducing those bass bombs.)
From "Toccata and Fugue in D minor" from Don Dorsey's Bachbusters album, to Van Halen's "1984," Mahler Symphony No. 8 as recorded on Telarc by Robert Shaw, Angela Hewitt's Bach cycle, ABBA Gold (and More Gold), E-Type's "Last Man Standing," Fun.'s "Some Nights," Pharrell's "Happy," Robin Thicke's "Blurred Lines," John Williams' "Star Wars: The Phantom Menace," The Eurythmics' "Sweet Dreams," and so on and so forth, everything sounded spectacular.
The only issue I noticed was that "Happy" and "Blurred Lines" caused a lot of rattles in the car along the door trim, but other tracks that were just as bass-heavy didn't. So there must be certain frequencies in the car that mess with the fit & finish.
So my whole-hearted recommendation is that anyone who is serious about good sound invest in the Ultra High Fidelity Sound System in the Model X. Using good source material, you won't be disappointed.