From someone that has been lucky enough to have never suffered any back issues I get that the seats are more supportive in the GTI but wouldn't that be offset by the firmer ride? The GTI does a really good job of smoothing things out considering what it is but I can't help think if it was that bad I'd rather have something with more SE suppleness? or does it not work like that?
I think that whatever the specific back problem is will be a factor in deciding whether or not a particular seat is comfortable and offers sufficient support in the right places. Lots of other factors also have a potential impact on whether what’s comfortable / supportive for one person will also offer sufficient comfort and support for another; e.g. a person’s height, build, weight, leg/arm/body length within their overall height.
I’ve had back issues for nearly 40 years and currently visit my chiropractor every 2-3 weeks to help maintain my flexibility to help keep my back-related issues under control.
I’ve owned a range of performance and non performance cars during my 40+ years of driving, some with very good, supportive seats and some less so. I found the sports seats in 1980’s/1990’s warm/hot Vauxhalls that I owned were very good; with good bolsters, infinitely adjustable lumbar support and extendable seat cushions. I also find the seats in VW’s performance models good.
IMO, there’s no substitute for taking an extended test drive - if possible over a number of days and for different length of journey of different road types, as that the only way the OP will know whether the seats in a GTI are right for them.