I found this very interesting. I was doing a little research and stumbled across this:
Q: It seems as though 3" exhaust is the popular choice these days. Is there any disadvantages? A: Yes! A stock horsepower car with 3" exhaust can lose noticeable low end torque and affect performance. Also, 3" pipe usually requires increased installation precision because of limited clearance issues. As a general rule, cars with less then 450hp should use 2.5" exhaust. Cars in the 450hp range could potentially benefit by a 3" crossover and still keeping 2.5" tails. Cars with 500hp + should utilize 3" or even larger tube size.