If the inspector knows what he is looking for or you have an emissions test yes the car will fail. And even technically putting a catted downpipe on can make you fail an inspection. It will give you a CEL but there are fixes for that.
I have a catted DP on mine with no CEL, I think most will not give one. My state has no inspection so I could have gone catless if I wanted to. For the catless pipes I think there is an adapter that you screw into the pipe that will fix the CEL. As far as getting a catted pipe, some come with 2 high flow cats in which case you are still technically legal, others like mine only have one cat in them, and you take a chance that if you do need to get inspected it might fail. Just got to know how tough your inspection process, and inspector are.
I have my 05+ LGT Perrin Catless Downpipe on my 08 WRX now, and it mated up just fine to the Ultimate Racing CBE. It did not mate up with the stock CBE, BUT if you had the adaptor for the downpipe that you could use a donut gasket on, then it would have worked. There was only maybe a 1/8-1/4" gap max so an adaptor would make it fit I believe.