Upgraded 2021 Model 3 with Highland Suspension
I figured I would add another anecdote to the topic. I replaced the front struts and rear shocks (not springs) on my 2021 dual motor. Holy cow... what a difference. I can't believe I waited two years since buying this car to make the upgrade. The ridiculous ride quality has been a pain point for me with this car; it was the only thing I was truly unhappy with and that made me miss my old Acura TLX. It rides so much better now. Bumps, harsh edges, and whoops are so much softer, and highways actually feel smooth now instead of constantly jarring. I can lean my head back against the headrest while doing 70mph and not have it constantly jolted around.
I literally felt the difference in less than 2 feet of driving, as I backed over the 1/2" ledge from the garage to the driveway. It was so much softer. Moreover, there previously was a constant rattling noise coming from my rear seat, and now I hardly hear it presumably thanks to the cabin not being jostled so much.
I'm glad Tesla fixed the ride, and I'm especially glad the new shocks can be retrofit on the first gen. But I can't imagine why Tesla tuned the ride so terribly on the first gen. Now I'm truly happy with this car and will be happy with it for a long time.
I was torn on whether to do the rear springs as well, but the shocks along made such a big difference that I'm glad I didn't go through the trouble and expense of replacing the springs. Compressing the old and new rear shocks by hand demonstrates the difference. The new shock rebounds in around one second and the original shock took more than 30 seconds(!) to rebound.
As far as the install, it wasn't bad. I did it alone in about 6 hours with all ordinary tools except the special thin-walled 13mm socket for the front strut upper mounts. Speaking of which, those were the only tricky part of the job. Getting the studs to line up perfectly to accept the nuts with the thin-walled socket was a PITA. The rear couldn't have been easier, maybe 30 minutes total.
TL;DR: if you're not happy with the stiff ride of the first gen, retrofit the Highland shocks. You won't regret it.