Agreed. No biggie. I only run my stock 19s because I have them, have the stock rubber on them with nothing else to do with the tires. Makes sense to me to burn up those tires on the track than to use my 20s.
I do believe that 19" set up is better for the track (road racing) than 20". This would be better if only for the fact that you can get competition R compound tires.
Many have gone to 18" from 19" on the GT3 for the track.