I bought the "modern spare" kit for the Model Y for a trip into a remote region -- and I had a flat. My review is kind of mixed.
I'll start with the biggest positive aspect of having a spare vs my understanding of what would have happened without:
* I took around an hour to change tires (will explain why it took so long below)
* then I drove 1.5hr to the nearest town where I could get a replacement
* everything was closed so I got a hotel
* the next day I made a dozen phone calls to find a shop with (approximately) the right size
So it wasn't exactly fast to get back on the road, but if I had been dealing with tesla roadside I believe the situation would have been:
* walk/hitch until I can get phone service
* wait for their tow truck
* they would tow me ~50 miles for free and I would have paid for another 30ish miles out of pocket
* they would have dropped me off at some tire shop, but I wouldn't have had easy access to a hotel and the shop might not have the size I need
So I think having the spare gave me easier access to hotel, food, and getting to the one shop in the area with approximately the right size tire.
The biggest failure for the modern spare kit is the included jack. The metal bent and it collapsed. It took two helpful passers by who loaned me their jacks and quite a bit of digging and elbow grease to get the car back up after the collapse. I'm very dissatisfied with the quality of the modern spare jack.
My other thought about the kit is that the spare is already quite large, enough so that I'm not convinced the space savings vs carrying a full blown 255mm wide replacement tire is any worse than carrying the "small" 145mm wide spare. The whole process of driving to a nearby town (when all the tire shops were already closed) and finding a shop that even had a replacement of the right size was very slow. If I carried a true full size spare I wouldn't had to go into town at all and would have saved nearly a day of time spent dealing with the replacement.
I could be wrong, but if I can pack my travel gear around the spare then I can probably tighten things up and pack around a full 255mm replacement. In fact for the rest of the trip I did carry the original 255mm tire in the car because discount tire wants to see the original to reimburse me since I had their insurance on the failed tire. (The way their insurance works when you're outside any area where you could reasonably reach one of their shops is they let you go to any shop and they reimburse you for the replacement).