I'm not entirely sure. I have seen one ST165 with those stickers, however, so if they're on more than one car(and I'm sure you're not talking about a white ST165 in Colorado Springs, CO), that might mean something. I was kind of curious as to what they are, but I know what you're talking about.
Edit: Did a little looking online... Toyota raced some sort of 4th-gen Celica on the GTO circuits; it was turbo, but I doubt it was a GT-Four. Actually, the Aeroform body kit for our cars was inspired by styling from the GTO race car. Maybe the stickers are a homage to that?