strictly speaking, an ins co would have a MAJOR problem declining a claim because of exhausts, unless that part was to blame for the accident/theft etc. If they were able to always use the non-standard argument for claims avoidance, no one would be paid out! non std hub caps, dealer fitted extras eg cd, factory fitted extras would all invalidate the ins cover. Also, they will only ever reimburse the cost of original not aftermarket, should any replacement be needed. However, if you declare non standard parts, many will decline.