After hacking the VGA box, you technically aren't running in VGA anymore. . The games are now running in RGB Scart mode at 15khz. There is no anti-aliasing going on. Some US games don't like this rgb mode, but I have only encountered 2 or 3 personally. I'm curious which driving games you say will not work with your hacked vga box?
There is also a trick you can try to get some of these other games to work with the VGA box. What you do is put the switch to TV mode, boot the dreamcast and wait about a second after you hear the startup sound end, you flip the switch to VGA. It may take a couple trys to get the timing right...what you are trying to do is trick the game into thinking its starting in tv mode, but then quickly switch it to vga mode just as it displayis the Sega License logo. You can also try using a boot disc.