As mentioned, you'll get a lot of opinions on this subject. In my opinion, the most cost effective solution is the Fullsac option. If you want a bit more performance without going to any extremes, go with their full Stage I option...the X pipe Header, 1.75" cores (you said you did not want loud), or the 2" cores (slightly louder, but not obnoxious), and the TTS Mastertune. The program for the ECM provided by Fullsac with the TTS is close enough without having to do a Dyno Tune. This option will give you about a 10% increase in both HP and TQ, but the main thing is it will give you better drivability...better throttle response and a more fun bike to ride. If you have basic tools in your garage, and a Laptop computer, you can do this work yourself for around $1100 US. You're in France, so I don't know about the exchange rates, customs charges, etc.