Just curious about your Duke. Is it the R model? Because, as far as I can see, the most useful mod that one can make to the basic Duke is to improve the suspension. Of course, I am a very slow rider, and I did re-gear to 15/42, but, the power seems adequate for everything but high speed stuff, and a few HP gained at big expense won't make almost any difference there. Also, although the bike protests when run at less than 3000 RPM or so, I never notice this, as it is so easy to avoid running the engine that slowly. In fact, I worry instead that screen dumps from my GoPro always seem to show my tach needle way up the RPM scale, and with 15/42 gearing 70 mph indicated is 5000 RPM. I don't ride freeways unless I have to and, I just take it on faith that a giant 690cc single can buzz along at 5000 RPM without fragging.
So, if you don't want more noise, and you are fixing to spend some more money, I would suggest reading up on suspension tweaks since I assume since you want more HP, you are already a fast rider, and, in that case, you should find the nonadjustable suspension kind of soft when you start to really push.