What state? I live in NJ and for OBDII and while they do not do a sniffer for OBDII, they roll a mirror under there real quick just to visually check to see if the cats are there. If not they have every right to have your car towed away at your expense. I personally would not chance it, swap your stock pipe back in for emissions day and swap back out afterwards. Or just buy a catted pipe and lose maybe all of 1 rwhp and then not worry about it at all. My buddy got pulled over in Wisconsin by a cop which does NO inspection and the cop took one look under his car and he got a BIG ticket and had to have the car inspected within 48 hours or even bigger fine. Then again I drove around for 7 years in NJ with an O/R pipe, got pulled over a few times for speeding and never had an issue.
2000 Mustang GT Steeda #0048
Just because I give you advice, doesn't mean I know more than you. Its just means I've done more stupid ****.