I have dreamed of putting 5-series BMW seats in my LC. Sadly the airbags and the way the seats communicate with the computer make this a potentially risky proposition (if even possible). I love the seats in my 335 m-sport and my wife's Macan S. Neither has upright seats, but both are very supportive and non-fatiguing for long trips.
As far as a solution, what I did with my LC is take it to a local upholstery shop to have the seats recontoured. The shop added additional padding to the front edge, which makes the seat slightly longer and higher, to better support the legs just above the knee. Then they built up the lumbar and the center-back portion of the seat to fill in the hole that all Japanese seats seem to have. This brings the back into a straighter, more upright position, straightening the neck and removing the curled, hunched-over feeling. I have been told that by pushing the head forward and curling the back, the seats perform better in crash tests. Why Japanese seats seem plagued by this and German seats don't, I am not sure. I have noticed that smaller people (< 6'), don't seem as affected by this type of seat. My theory is that the smaller you are, the less your head is pushed forward, so smaller people are in a more natural position.
Anyway, for less than $300, I was able to make the driver seat much more comfortable and supportive, especially noticeable on long trips. It may take a couple iterations to get it just right, but I have done this with 2 different vehicles to great success.