the 15s have a smaller circumference and less weight. tire circumference, final drive, and gear ratio all play into acceleration. Since changing gear ratios is expensive and time consuming (or not possible with the getrag in the SVT, swapping to 15s is easier and relatively cheap. SVTs that autocross frequently do this because 1, 2, and 3 are all tall gears, and having the 15s helps get better times.
in the turbo car world, people tend to put not only wider (for traction) but taller wheels/tires on to make a larger circumference, effectively making the gearing longer, meaning I can now get up to 40 instead of 35 in 1st (not a real example, just throwing numbers out).