I have been obsessed with a quiet car and have had the pleasure of driving several models of Lexus as well as a VW Phaeton which was tank quiet. As many have also commented on the forums, the interior noise from the Tesla comes in several forms. Helpful to think about when planning sound modification. It all boils down to time, money and how much you want to take apart your car.
1. Engine noise - virtually nonexistent in the Tesla. However, the drive unit does cause some hum which can be minimized folding down the rear seats, using the well cover, and using the parcel shelf. Dynamat (or equivalent, Dynamat is damn expensive and there are way cheaper alternatives) in this area under the carpet can help
2. Road noise - probably the largest area of improvement. First and easiest solution is to put a second set of mats in the front and rear over the stock mats. Next would be to Dynamat the doors which is relatively easy as instructions to remove the door panels are available on TeslaTap. Just be careful putting too much Dynamat as there is a law of diminishing returns and all you are doing is adding weight, cost, and time. Putting Dynamat under the carpet is too much work and would require significant labor and disassembly-probably not worth it.
3. Window noise - most luxury cars have double pane glass which we do not. This may be an option in future models. Nothing you can do about this. I have heard that window tinting can sometimes reduce noise transmission
4. Wheel well noise - another significant area of improvement. Certain tires will reduce road noise - see tire rack for more details. Also helpful to paint the wheel wells with a paste or liquid sound dampener. I plan on coating the wheel wells with Second Skin Spectrum in later April when I do my winter tire changeover and paint the break calipers.
In the end, you can go completely nuts with sound isolation if that is your goal.....most people weigh the cost/benefits. One of the challenges with our car is finding an installer who feels comfortable taking it apart and putting it back together. Most of the projects are easy DIY if you have the interest and basic tools. Just my 2 cents. Look forward to other peoples thoughts and experiences.