My input to this thread is based on information that is stale, but hopefully it will help with your decision.
I have a P85+ with the Sound Studio package and the car went into Tesla service for some work and I had the opportunity to drive a P85 with a standard sound system for about a week. I believe this was before the new Codec's were implemented by Tesla to account for the panoroof vs. the standard roof. Both cars I drove were basically the same in the interior (pano roofs, leather, Alcantara - basically loaded interiors).
At the time I primarily was using Slacker as source material, so my thoughts reflect how the cars handled Slacker material. Compared to the Sound Studio (SS) the standard system lacked image clarity and separation. Also the bass response of the standard system is not as good as the sound studio. The low end is not really present in the standard system. Even if you go in and set the low EQ to +12 dB, it isn't really providing the low end support like the SS does.
That being said, the SS also doesn't have great bass response (although it is much improved over the standard audio). While the bass is present with the SS, it is not as tight and the response curve is not as smooth as it could be. While the bass notes may be there, some are softer than others and the notes sound mushy. The SS is a fine upgrade from the standard audio package, but the potential is still there for improvement.
From a subjective standpoint I would describe the difference between the standard and SS audio like this:
The standard system is like listening to music with gauze or a bit of cotton in your ears. When you upgrade to the SS the gauze is removed and the music clarity and separation is much improved. With the standard system the bass really isn't there. It's kind of like listening to a orchestra with a very small bass section in comparison to the rest of the orchestra. Notes are missing, especially as you get into lower bass notes. With the SS system the bass is present, but it's still like listening to bass with cotton stuffed in your ears. There isn't as much tone to the bass as there should be, it's muddy and not that well defined and notes aren't necessarily clear and sharp.
I ended up doing an aftermarket upgrade and am very happy with the results.
GTHill, since you have listened to the SS and are happy with the results, I think that you would be disappointed over time with the standard system, primarily from a bass response and from a clarity and separation standpoint. But remember, I have not heard the standard system recently and I upgraded my own system before the 5.8 software release, so I don't know how much Tesla has been tweaking the Audio stuff in their software. I'm sure they have been doing it, but I can't say how much that impacts current cars.
Best thing to do is to find some music you are very familiar with and rip it to AIFF format at 44.1 kHz and with 16 bit encoding. Load the music on a USB stick and find some cars to do side by side comparisons. Also I'd recommend testing with Dolby encoding off and with all EQ controls set to flat at first (also confirm that the front/rear left/right spatial controls are set to neutral).
From there start playing with the controls and see if you can get to a point that you think is good. Then compare the other system from a base line level and then tweak to see if you can get any better outcome than you did from the first car. Hope that helps!