86 decibels is the German sound limit i'm told. There's an iPhone app decibel meter pro - walking around holding it up to the RT, GTL, the C650 is reads 86 decibels about 1 foot behind each exhaust pipe.
my take is 92 decibels is obnoxious, 96 is retarded loud, etc. every 3 dB's is like tripling the intensity.
would a 250 vespa be quieter? - probably