In August 2015, I changed the original exhaust system of my Honda CBR 125, with a complete Akrapovic exhaust system. The new exhaust is carbon made, and originally it had a db-killer to reduce the sound/noise. But of course, the sound is much more better if you remove it. If you decide to put this exhaust system on your motorcycle, please remember that you cannot ride on normal streets anymore (it does not satisfy the exhaust gas limitation rules of most Countries).
Talking about the sound, I think it improved a lot. The original sound was so weak, but after changing to this Akrapovic exhaust system, the motorcycle sounds much more like a Moto3 motorbike. Have a look at the following video. I am sorry for the quality, I will try to take a better one in the next days.
More details on this product can be found on the Akrapovic official website. In the website, you can find also the installation manual. The installation is very easy and it requires just 20-30 minutes, supposing that you have all the necessary tools.
According to the official data provided on the Akrapovic website, this exhaust system increases both power (+0.6kW at 9170rpm) and torque (+1.0Nm at 6670rpm). Considering a total engine power of about 10kW, it means an increase of about 5%. Of course, it is not so much, but the sound is awesome. The only bad point is the price: I paid around 70,000 Yen (500 Euro) for the complete carbon exhaust system. But I am very satisfied.