seems weird that your a3 sportback is that low for 225/40/18, my 2014 3 door a3 is 38 front and 35 back tyres and it feels perfect and that what is recommended on sticker inside door for mines, i used to put my own pressures in but once i started using the pressure on sticker it felt much better to drive.
You would think the sportback a3 would be higher or atleast the same as 3 door on 18” wheels