Generally exhaust has a smaller impact than other places in the power train, but it definitely has an impact and moreso in a motorcycle vs. a car.

Generally, freer flowing exhaust will make more horse power however if you go too crazy (think open headers) it can decrease low end torque.

For a motorcycle I say go for whatever exhaust you want and you'll be fine. Motorcycles are high reving small motors that could use the extra air flow from the exhaust

Aftermarket exhausts that allow more airflow improve performance, not hinder it. But if you improve your exhaust rate, then you also need to improve your air and fuel flow. The right way to do it is to get an exhaust, a high flow air filter, and re-jet your carbs, or retune your injection. Then you maximize all of the essentials all at once and you pull more HP out of the same engine at the cost of noise and emissions.

It's not usually enough power to make a big difference, and it can raise the noise level to a point where you might not like how it sounds on longer rides. Some can provide weight reduction, but again not a lot.

most of the back pressure that effects performance comes from the cat, not the pipe. Expect minimal horsepower from a bolt-on. A cat delete would make a difference, but would also cause your bike to fail inspection (depending where you live)