I agree that having the tire measured wheel width matching the wheel width would be ideal, but the ideal match would be 225/40R19. 225s are way undersized for our cars power. 225s should provide better feeling and tun in response but would sacrifice maximum grip. The ideal solution would be to find a 9.5" wheel with the correct offset and mount 265s or 275/30R19, but that would cost a lot of money and I don't think any of the mainstream wheel manufactures make them with the correct offset. So putting 245s on the factory wheels fits most peoples budget and performance needs.
For my autocross tires I'm stuffing 255/40R18s on 8" wide wheels due to class rules and just switching from 235s to 255s kicked off 2 seconds on a 60 second course. When these tires get to the end of life, I'm going to move to even wider 265s.
Are you running the stock wheels for autocrossing? How wide and tall can we go with the stock wheels? The front strut clearance doesn't look like there's much room??
* 2017 Fusion Sport, 12.7 @ 108 mph tune-only (early tune of mine, bad trans settings), full trunk w/ lunch, and dirty paper air filter
* 2014 SHO non-PP, 12.4 @ 111 mph tune-only, all day long with dirty paper air filter and worn out 30k RS-As