Hey Jkat, I think I would try to see if an upholstery shop could possibly retrofit the seats with firmer foam padding, or additional padding. Like you said, I think your options are somewhat limited. If you can deal with the unsightly support cushions that they sell for additional lumbar support, you could go that route. If its financially feasible for you, trade out for a GS 450h. I'm almost certain the GS will have more supportive seats. Keep us posted on what happens