In my last car, a 2012 Skoda Octavia vRS, I fitted a full size 225/40-18 tyre/wheel by using a home-made false boot floor with the carpet on top. I used a sheet of 10mm (or maybe 12mm) plywood supported on blocks of wood glued to the underside. No problems in 6 years of ownership.
I had done similar in the car before that, a 2005 Civic Type-S. I haven't bothered in my current 2018 Leon FR & have an 18" space-saver.