the 15s are good because there is less wheel weight, as opposed to 17s. 17s are larger wheels, and weigh the car down more. 15s are smaller "wheel" so there is less weight and more sidewall.
Now in the case of 205's, that's not always good. 205's are kind of small in width (205 being the width of the tire in millimeters). 205s wont have as large of a "foot print" and are more narrow, giving superior driving ability in cases such as snow where a thin tire is a good thing. In cases such as performance, thin is not good, as it doesnt allow for as much GRIP, as well, as, braking. It's a trade off, but if you're planning ultra-high performance where forced induction is involved, grip is more important than a few pounds saved, and spinning your tires off the line is a big no-no.
Thanks for responding. The reason I mentioned 205 was because I think thats the stock tire size on a 15 inch stang wheels. I just saw BFG has 225 50 15's that fit 6-8" wheels. That would probably be a better thing hey?