My problem with the X3 (and with the Q5, and the sportseats in the XC60) is the heavy bolstering on the narrow bottoms. I have to drive with my legs squeezed together. I don't mind hard seats though. Firmer the better, at least for my body.Before I got the Q5, I cross shopped the X3 and GLC. The X3 was the most fun to drive. But the leather seats were like sitting on plywood. That was a deal killer. The GLC has great seats (the MB Tex is unusually comfortable). However it was the mushiest and least sporty of the three. I drove the GLC recently and it is comfortable but really lazy is the best way to describe it.
I drove the XC60 T5 with Polestar already installed from the factory. It is definitely more lively with the Polestar upgrade but not sure a requirement.
Seating is such a personal issue though. What works for one driver does not work for another.