Fuel
You asked what a “straight piped chopper” was and you were supplied the anwser, true it is not a gixxer but it was the correct reply to your question
Back to the OP, there is one manufacturer of mufflers that uses a “mini can” stuck on the side of the main can, because this smaller can a differant time delay through it , then at certain engine revs, the gas pulses and travelling up it and the negative pressure pulses going back down it, fill in the gaps between the pulses in the main can and so reduce noise.
It is not designed to reduce overall noise though, just to get it legal at the standard measuring conditions, however I would think you could contact almost any bike exhaust system manufacturer and get useful information.
I cannot remember the name of them, there are so many of them, do a search for manes like Remus, Yoshimura, Supertrapp(this is a tunable system so there could be something here for you, Motad,Cobra,Laser, Predator.
If you want a stealth machine then a Gixxer is not it, the only reason that people fit aftermarket exhaust sytems on them is to make them louder and imrove the power delivery.
If you are truly inot a quiet machine then you would have to look for a differant type of machine, the VFR800 is very quiet below 8k revs, and the STX1300 is a quiet machine, but the quietest would probably be a Goldwing of some description, or a variant with the same engine.
Other quiet machines would include the “super scooters” such as the Burgmans and the like.