Im a little surprised as the seats on my previous mg4 had been very comfortable over long journeys. They are not immediately comfortable when firsted used in the mg4, but no aches or pains after steppinf out of 2-3 hours in the car.
the S5 seats seem to be a step up from the mg4 in terms of level of padding/ materials/ lumber support. However I have not yet done a 3 hour journey in the S5.
I do suffer with back pain, and have had alot of cars in the past, which on initial first use the seats seem good , but after 2-3 hours they become uncomfortable in use.