It is my understanding just installing or modifying the muffler will not gain any noticable performance change. I believe to gain power you need to improve air flow in and out of the engine. Plus rejetting the carbs to rebalance the fuel air mixture.
Most seem to feel if you only change exhaust rejetting is not required. I see some that feel any change in air flow in or out of the engine requires rejeting. Here is some reading on exhaust.
Here is a picture of a three hole mod on a v star 650.
This one claims no rejetting. I would consider aftermarket.