My GC STI front was 19mm and rear was 20mm which was really good for a long time but after a while...
...I upgraded to adjustable 22mm front and adjustable 24mm rear, to keep a similar balance because the car was good, and adjusted them until I had the right combination of flatness, turn-in, oversteer, understeer for my driving style in tarmac autocrosses. They're set on softer of the settings 21.5mm front and 23mm rear. When I set the fron to 22.5mm I got a appreciable amount of understeer. When I set the rear to 24mm I got oversteer. When I set them both at the same time I got both understeer going into a corner and oversteer coming out of the same corner and it was unmanageable and unnerving.
I running standard STI V4 springs (which are softish).