I'm curious, just what sort of modifications do you have in mind. I've driven Volvos for years and Volvo is famous for it's excellent seats.
When I bought my M3 a year ago, I was pleasantly surprised to find the seats were at least as good as Volvo's...maybe even better. I don't do
"full bore starts" all that often, but when I do and have a passenger, I never here complaints about the seats.