I'd still expect a couple of horsepower and torque. Might not be much but it's something for $300.dyno for a 125cc? top speed is around 65mph...doubt you can get that much more on a dyno test...maybe if you add a turbo it would be worth it.
With such little farings and covers on the grom getting to all the bolts are so easy. I know this video is fast lapsed but it wouldn't take long at all to install the exhaust.