How would a stock 2000 Legacy GT 2.5 stand up to a 2005 Chevrolet Cobalt 2.2 ecotec in a race? What about the Legacy against a 2000 Maxima SE. Also if you could let me know about how fast the GT 2.5 is and how well it handles that would be great thanks.

The Legacy is heavier than both those cars, it would maybe pull on them off the start, but not for long.

The GT handles great though and is pretty quick, it just ain's a track car by any means.
If you want a track car, you want an Impreza, Subaru designed them as a race bred car.