yea thats the main thing here, they can say anything they want is illegal or not compliant! its up to you to prove them wrong! honestly i dont think talking to his seargent is going to do any good! i mean if you were his seargent and someone came to you and complained about the way in which a police officer under your command defected your car or spoke to you, would you take any notice!
carlo is right, if you change somthing on your car that may get you into trouble with the fuzz, then get it engineered or get written documentation for it, so when he says your exhaust is illegal you can say to him, officer i have written documentation of the validity of (enter Modification here) and theres nothing he can do about it!
treat him with respect even if he does return the favour and save urself the hassle!