Seats are very personal things, I find. In my case, I have had back trouble for years, so I tend to be very fussy about seats (not only in cars, but furniture as well). It is not always apparent what makes a good seat, or what will make an uncomfortable seat better for a particular person.
Before my Tesla, I drove Mercedes Benzes for decades, several different models, and never had one with seats I truly liked, despite their vaunted design supposedly created by "orthopedic surgeons," as the ads used to say. My solution was to get an after-market seat insert that gave me more padding and had a proper lumbar curve. The ones I have had (I have had several over the years) are by Sacro-Ease -- here is a link to the Amazon page for them:
Amazon.com: sacro ease
My Model S (which has the standard leather seats, not the next-gen seats) is no exception to my discomfort, unfortunately, so I got a new Sacro-Ease for it, which helps me to tolerate long rides in the car. (I do find I can occasionally tolerate using my Model S without the Sacro-Ease for a while, and once made a 3-hour trip without the cushion so that I could use the seat heater. But I always end up going back to using the extra cushion after a while.)
My spouse, who normally is not at all fussy about car seats, also finds our Model S driver's seat to be uncomfortable, but says the front passenger seat is better. So it could be that there is some detail of the driver seat's construction that catches us just wrong.
Before I ordered my Model S, however, I had rented one at the San Francisco airport, to try it our for a day. The one I got was a very old Model S -- a late 2012 P85. I was in and out of the car over the span of 24 hours, with a couple of long drives. My back never bothered me in that car. Maybe it would have if I had driven it for longer stretches, not sure. So I had sort of hoped that the seats in my 2015 Model S would also be comfortable for me, based upon my rental experience. But they seem to be too hard on my butt. (In my case, I do not seem to have the backrest issues that the OP is describing, but all the components work together on one's spine, so it might be too easy to say that the backrest is OK for me.)
I tried the second generation seats on one of my test drives before ordering, and did not care for the snug bolstering. But I cannot say that I gave them a good comparison test.
Also, i recently got a Model X loaner when my car was in for service. I drove it a fair amount over the span of 30 hours or so, and did not like the Model X seats. Initially I drove in the bare seats. But after a while, I was sore and had to use my Sacro-Ease in the Model X as well.
So, people who find their seats to be uncomfortable might try the type of after-market seat I have, or any of several others available. I got mine at a Relax-the Back store, which is a chain. They helped me choose the right model for the car, and would have exchanged it if it did not work out for me.
Another alternative I used once on a previous car, with marginal success, is to have an upholsterer rebuild the seat. They might be able to adjust the padding to suit your comfort. (In my case, I was trying to rehabilitate an old car's seat that was starting to sag.)
Oh, and as for BMWs, once many years ago, I actually bought a used BMW 5-series seat and installed it in my mid-size Benz. It was better, but I could never get it positioned quite right and I was worried about the structural integrity of how I attached it, so I removed it. I tried after-market Recaro seats, too, but found them, to be too hard.
Good luck to anyone who is having seat comfort issues. There is more art than science to being comfortable, I suspect, and despite trying for many, many years, i have yet to find the secret!
