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