The markings are stamped so that they are not too conspicuous.
The guys at the shops probably have to tell you that from a legality point. this is so you can't blame them if you ever get checked.
I put the Speedmaster slash cuts on my America, improved the look and sound. The only time it will present a problem is if the cops pull you over for noise, you can assess the likely hood of this by the number of loud bikes in your area.
Down here the only way you get hauled in for noise is if you upset your neighbours and they report you.
Just do it and continue to enjoy your ride.
