Thanks a lot anyway Bill and all the other guys! I am now thinking, maybe I am just looking a gift horse in the mouth. This Zard that came with the bike worth around EUR 2.200 new, while I paid the whole bike EUR 6.500. So maybe I could just try to remove the CAT out of it (it has a honeycomb-type CAT directy pressed in the silencer entrance) and be happy with what I have, rather than to push luck with improvizing with 3 pipes
Update: I removed the CAT from the Zard silencer. Left the DB Killer in place. Made a difference in sound, not that much at idle but much more noticable on the road. The sound changed from "big scooter" to "old fighter airplane". Worth the effort.
PS.
When I removed the silencer, ofcourse I could not resist to fire it up with 3-1 manifold only. My face turned into a big smile without me even noticing it Now THAT is the sound I ask for, literally an old Cummins with straight pipes :=D Too bad it bangs a lot and shoots flames like a torch so I guess it would kill the exhaust valves which is not worth the risk, not to mention it would probably screw the performance.