I'd prefer a bit adjustment on the cushion actually, my FTO lets me move the front and rear of the cushion up and down. Bringing the front up to support the legs is very comfortable.

Funnily enough comfiest seats I ever used were in an old Mk3 Fiesta Si

 

I've got a couple of supports from "The Original McKenzie Lumber Roll" available from Medipost 0161 678 0233, and other places I'm sure.

 

I had a bad back when i first had the car , physio thought the seats were poor and recommended more lumbar support.
Certainly on a 51 plate car the rear panel of the seat is only attached top and bottom and can have extra padding stuffed in, which is what i d did ten years ago, I think its still there !