This assertion is completely wrong.
A 6" wide wheel isn't designed to mount a specific tire, it is the other way around. A whole range of tires will fit and perform properly on a 6" by 17" wheel, even as small as 170/60.
So you are saying that the tyre manufacturers make their tyres to fit whatever rim size the motorcycle designers take a fancy to.....interesting.
I'm also not convinced that going a narrower tyre is a smart option. I believe it's certainly not the safest option as you may run the risk of the bead unsealing, and I am wondering what your insurance company would say about your deliberate negligence in not adhering to manufacturers specifications?
Also, interestingly, my BMW manual states that the vehicle is considered unroadworthy with any untested tyre size fitted, that would be a good question to ask your registration and Insurance people, but more importantly from the manual, "Wheel size is very important as a parameter for the chassis and suspension control systems ABS and ASC. In particular, the diameter and the width of a vehicle's wheels are programmed into the control unit and are fundamental to all calculations. Any change in these influencing variables can have serious effects on the performance of the control systems." But hey, those are the Germans and they are a bit anal about this stuff, I am sure the Italians have said to their computer systems, heck, the owners will do whatever they want with our finely engineered piece of equipment. They know more than us so just deal with it.
However, lots of luck, let us know how it all turned out.