I received a modified exhaust ticket on my E60 M5, which just had the DINAN rear section. Cop was a complete a$$hole like your cop - they seem to enjoy making a fuss out of the fact that you drive a nice car that sounds great. Some sort of inferiority complex that their Crown Vic's dont sound as good..
They made me go to this referee station where they tested my exhaust levels, and it was past the legal limit.
Car was totaled a couple of weeks later, so the citation was ultimately dropped. That's a sticky situation if he's threatening you on second offense. Sounds like someone that needs to have their badge taken away. I'm so sick of hearing about cops and their power trips. Two months ago I got an exhibition of speed ticket in LA because my exhaust was too loud, which is a misdemeanor, and it cost about four grand including the attorney and court costs. They make it really difficult to enjoy a nice car here, even when you aren't doing anything wrong.