Actual Gas milage/GT or STI- seriously.

2 things guys. 1- What's the true ave. gas mileage for the GT. I just dont believe that it's 21 for an 8 cyl. American car. Also, how does it feel to see a car like an STI fly by on the highway, and know that there's no way to handle/perform at that level? I want a GT sooo badly, but people assure me that I wouldn't be happy with the performance. Thanks.