I think it'd be louder than you think. An open ended turbo screams very loudly indeed, and chucks out the odd flame or two on lift off.
It might be a good way to inimidate the VoSA lot that do roadside emissions tests though. They'd need kevlar suits and asbestos gauntlets
"Just rev it up gently please sir whilst I attend to the lambda probe attached to the tailpipe."