I think you will find if all you do is an aftermarket exhaust you will mostly just gain noise. Some would say a cool pleasing sound but little change in performance. The V Star 650 has a carburetor so to change fuel mix you would need to rejet. But for many exhaust's if that is all you do many do not change anything more.
To gain a noticeable (Maybe 10%) Horsepower change you would need to change Airflow in, rejet, and exhaust.
If you should buy a fuel injected bike like for example a 950 V Star then an electronic fuel controler could be changed. The story is the same on the exhaust and air box.